If of any help, this is the complete journal log associated with a rejected logon event:
Nov 13 12:28:07 sirius sshd[789]: debug3: fd 4 is not O_NONBLOCK
Nov 13 12:28:07 sirius sshd[789]: debug1: Forked child 2159.
Nov 13 12:28:07 sirius sshd[789]: debug3: send_rexec_state: entering fd = 7 config len 1050
Nov 13 12:28:07 sirius sshd[789]: debug3: ssh_msg_send: type 0
Nov 13 12:28:07 sirius sshd[789]: debug3: send_rexec_state: done
Nov 13 12:28:07 sirius sshd[2159]: debug3: oom_adjust_restore
Nov 13 12:28:07 sirius sshd[2159]: debug1: Set /proc/self/oom_score_adj to 0
Nov 13 12:28:07 sirius sshd[2159]: debug1: rexec start in 4 out 4 newsock 4 pipe 6 sock 7
Nov 13 12:28:07 sirius sshd[2159]: debug1: inetd sockets after dupping: 3, 3
Nov 13 12:28:07 sirius sshd[2159]: Connection from 192.168.8.3 port 48708 on 192.168.8.3 port 50400
Nov 13 12:28:07 sirius sshd[2159]: debug1: Client protocol version 2.0; client software version OpenSSH_7.4p1 Debian-10+deb9u4
Nov 13 12:28:07 sirius sshd[2159]: debug1: match: OpenSSH_7.4p1 Debian-10+deb9u4 pat OpenSSH* compat 0x04000000
Nov 13 12:28:07 sirius sshd[2159]: debug1: Local version string SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u4
Nov 13 12:28:07 sirius sshd[2159]: debug1: Enabling compatibility mode for protocol 2.0
Nov 13 12:28:07 sirius sshd[2159]: debug2: fd 3 setting O_NONBLOCK
Nov 13 12:28:07 sirius sshd[2159]: debug3: ssh_sandbox_init: preparing seccomp filter sandbox
Nov 13 12:28:07 sirius sshd[2159]: debug2: Network child is on pid 2160
Nov 13 12:28:07 sirius sshd[2159]: debug3: preauth child monitor started
Nov 13 12:28:07 sirius sshd[2159]: debug3: privsep user:group 117:65534 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: permanently_set_uid: 117/65534 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: ssh_sandbox_child: setting PR_SET_NO_NEW_PRIVS [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: ssh_sandbox_child: attaching seccomp filter program [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: list_hostkey_types: ssh-ed25519,ssh-rsa,rsa-sha2-512,rsa-sha2-256 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 20 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: SSH2_MSG_KEXINIT sent [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 20 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: SSH2_MSG_KEXINIT received [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: local server KEXINIT proposal [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: KEX algorithms:
curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: host key algorithms: ssh-ed25519,ssh-rsa,rsa-sha2-512,rsa-sha2-256 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: ciphers ctos:
chacha20-poly1305@openssh.com,
aes256-gcm@openssh.com,
aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: ciphers stoc:
chacha20-poly1305@openssh.com,
aes256-gcm@openssh.com,
aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: MACs ctos:
hmac-sha2-512-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,
umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,
umac-128@openssh.com [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: MACs stoc:
hmac-sha2-512-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,
umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,
umac-128@openssh.com [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: compression ctos: none [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: compression stoc: none [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: languages ctos: [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: languages stoc: [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: first_kex_follows 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: reserved 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: peer client KEXINIT proposal [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: KEX algorithms:
curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256,ext-info-c [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: host key algorithms:
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,ssh-ed25519,ssh-rsa,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: ciphers ctos:
chacha20-poly1305@openssh.com,
aes256-gcm@openssh.com,
aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: ciphers stoc:
chacha20-poly1305@openssh.com,
aes256-gcm@openssh.com,
aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: MACs ctos:
hmac-sha2-512-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,
umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,
umac-128@openssh.com [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: MACs stoc:
hmac-sha2-512-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,
umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,
umac-128@openssh.com [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: compression ctos: none,
zlib@openssh.com,zlib [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: compression stoc: none,
zlib@openssh.com,zlib [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: languages ctos: [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: languages stoc: [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: first_kex_follows 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: reserved 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: kex: algorithm:
curve25519-sha256@libssh.org [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: kex: host key algorithm: ssh-ed25519 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: kex: client->server cipher:
chacha20-poly1305@openssh.com MAC: <implicit> compression: none [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: kex: server->client cipher:
chacha20-poly1305@openssh.com MAC: <implicit> compression: none [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: expecting SSH2_MSG_KEX_ECDH_INIT [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 30 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_key_sign entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 6 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_key_sign: waiting for MONITOR_ANS_SIGN [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive_expect entering: type 7 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 6
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_sign
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_sign: hostkey proof signature 0x55f0b9859a80(83)
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 7
Nov 13 12:28:07 sirius sshd[2159]: debug2: monitor_read: 6 used once, disabling now
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 31 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 21 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: set_newkeys: mode 1 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: rekey after 134217728 blocks [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: SSH2_MSG_NEWKEYS sent [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: expecting SSH2_MSG_NEWKEYS [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 7 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 21 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: SSH2_MSG_NEWKEYS received [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: set_newkeys: mode 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: rekey after 134217728 blocks [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: KEX done [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 5 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 6 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 50 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: userauth-request for user backuppc service ssh-connection method none [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: attempt 0 failures 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_getpwnamallow entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 8 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_getpwnamallow: waiting for MONITOR_ANS_PWNAM [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive_expect entering: type 9 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 8
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_pwnamallow
Nov 13 12:28:07 sirius sshd[2159]: debug2: parse_server_config: config reprocess config len 1050
Nov 13 12:28:07 sirius sshd[2159]: debug3: auth2_setup_methods_lists: checking methods
Nov 13 12:28:07 sirius sshd[2159]: debug1: authentication methods list 0: publickey
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_pwnamallow: sending MONITOR_ANS_PWNAM: 1
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 9
Nov 13 12:28:07 sirius sshd[2159]: debug2: monitor_read: 8 used once, disabling now
Nov 13 12:28:07 sirius sshd[2159]: debug2: input_userauth_request: setting up authctxt for backuppc [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_start_pam entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 100 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_inform_authserv entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 4 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_auth2_read_banner entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 10 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive_expect entering: type 11 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 100
Nov 13 12:28:07 sirius sshd[2159]: debug1: PAM: initializing for "backuppc"
Nov 13 12:28:07 sirius sshd[2159]: debug1: PAM: setting PAM_RHOST to "192.168.8.3"
Nov 13 12:28:07 sirius sshd[2159]: debug1: PAM: setting PAM_TTY to "ssh"
Nov 13 12:28:07 sirius sshd[2159]: debug2: monitor_read: 100 used once, disabling now
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 4
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_authserv: service=ssh-connection, style=, role=
Nov 13 12:28:07 sirius sshd[2159]: debug2: monitor_read: 4 used once, disabling now
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 10
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 11
Nov 13 12:28:07 sirius sshd[2159]: debug2: monitor_read: 10 used once, disabling now
Nov 13 12:28:07 sirius sshd[2159]: debug3: auth2_setup_methods_lists: checking methods [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: authentication methods list 0: publickey [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: Unrecognized authentication method name: none [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: userauth_finish: failure partial=0 next methods="publickey" [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 51 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 50 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: userauth-request for user backuppc service ssh-connection method publickey [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: attempt 1 failures 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: input_userauth_request: try method publickey [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: userauth_pubkey: test whether pkalg/pkblob are acceptable for ED25519 SHA256:zNt1fYSj/OdaqgbgzDWwUMGV2l9GmjaM6FTmlRuIbzg [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_key_allowed entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 22 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_key_allowed: waiting for MONITOR_ANS_KEYALLOWED [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive_expect entering: type 23 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 22
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_keyallowed entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_keyallowed: key_from_blob: 0x55f0b9867a10
Nov 13 12:28:07 sirius sshd[2159]: debug1: temporarily_use_uid: 200/200 (e=0/0)
Nov 13 12:28:07 sirius sshd[2159]: debug1: trying public key file /usr/local/BackupPC/.ssh/authorized_keys
Nov 13 12:28:07 sirius sshd[2159]: debug1: fd 4 clearing O_NONBLOCK
Nov 13 12:28:07 sirius sshd[2159]: debug1: matching key found: file /usr/local/BackupPC/.ssh/authorized_keys, line 1 ED25519 SHA256:zNt1fYSj/OdaqgbgzDWwUMGV2l9GmjaM6FTmlRuIbzg
Nov 13 12:28:07 sirius sshd[2159]: debug1: restore_uid: 0/0
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_keyallowed: key 0x55f0b9867a10 is allowed
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 23
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 60 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: userauth_pubkey: authenticated 0 pkalg ssh-ed25519 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: Postponed publickey for backuppc from 192.168.8.3 port 48708 ssh2 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: receive packet: type 50 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: userauth-request for user backuppc service ssh-connection method publickey [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: attempt 2 failures 0 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: input_userauth_request: try method publickey [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: userauth_pubkey: test whether pkalg/pkblob are acceptable for RSA SHA256:/TFnCvwpyL6W0Z4I71jw9u3eY4rWqB9lnHVSx3mdYNM [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_key_allowed entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 22 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_key_allowed: waiting for MONITOR_ANS_KEYALLOWED [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive_expect entering: type 23 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: monitor_read: checking request 22
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_keyallowed entering
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_keyallowed: key_from_blob: 0x55f0b985d0d0
Nov 13 12:28:07 sirius sshd[2159]: debug1: temporarily_use_uid: 200/200 (e=0/0)
Nov 13 12:28:07 sirius sshd[2159]: debug1: trying public key file /usr/local/BackupPC/.ssh/authorized_keys
Nov 13 12:28:07 sirius sshd[2159]: debug1: fd 4 clearing O_NONBLOCK
Nov 13 12:28:07 sirius sshd[2159]: debug2: user_key_allowed: check options: 'ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBIG1jFCDbjX1AuKEpi3U0rPtxW79kZwcEAzN+shbnni Generated on 181113.1203 for backuppc@sirius\n'
Nov 13 12:28:07 sirius sshd[2159]: debug2: user_key_allowed: advance: 'AAAAC3NzaC1lZDI1NTE5AAAAIBIG1jFCDbjX1AuKEpi3U0rPtxW79kZwcEAzN+shbnni Generated on 181113.1203 for backuppc@sirius\n'
Nov 13 12:28:07 sirius sshd[2159]: debug1: matching key found: file /usr/local/BackupPC/.ssh/authorized_keys, line 2 RSA SHA256:/TFnCvwpyL6W0Z4I71jw9u3eY4rWqB9lnHVSx3mdYNM
Nov 13 12:28:07 sirius sshd[2159]: debug1: restore_uid: 0/0
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_answer_keyallowed: key 0x55f0b985d0d0 is allowed
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_send entering: type 23
Nov 13 12:28:07 sirius sshd[2159]: debug3: send packet: type 60 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug2: userauth_pubkey: authenticated 0 pkalg ssh-rsa [preauth]
Nov 13 12:28:07 sirius sshd[2159]: Postponed publickey for backuppc from 192.168.8.3 port 48708 ssh2 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: Connection closed by 192.168.8.3 port 48708 [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: do_cleanup [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug3: PAM: sshpam_thread_cleanup entering [preauth]
Nov 13 12:28:07 sirius sshd[2159]: debug1: monitor_read_log: child log fd closed
Nov 13 12:28:07 sirius sshd[2159]: debug3: mm_request_receive entering
Nov 13 12:28:07 sirius sshd[2159]: debug1: do_cleanup
Nov 13 12:28:07 sirius sshd[2159]: debug1: PAM: cleanup
Nov 13 12:28:07 sirius sshd[2159]: debug3: PAM: sshpam_thread_cleanup entering
Nov 13 12:28:07 sirius sshd[2159]: debug1: Killing privsep child 2160
Nov 13 12:28:07 sirius sshd[2159]: debug1: audit_event: unhandled event 12
root@sirius:~#