sid: ssh session hangs when logging in

[Mike Kupfer <m.kupfer@acm.org>]

I recently set up a VirtualBox guest that is running Sid.  I can ssh
from it to the host.  But if I connect to my employer's network (using
OpenConnect), I cannot connect to any systems on that network.  Using
"ssh -v" I see

  ...
  debug1: Enabling compatibility mode for protocol 2.0
  debug1: Local version string SSH-2.0-OpenSSH_6.7p1 Debian-6
  debug1: Remote protocol version 2.0, remote software version OpenSSH_6.0p1 Debian-4+deb7u2
  debug1: match: OpenSSH_6.0p1 Debian-4+deb7u2 pat OpenSSH* compat 0x04000000
  debug1: SSH2_MSG_KEXINIT sent
  debug1: SSH2_MSG_KEXINIT received
  debug1: kex: server->client aes128-ctr umac-64@openssh.com zlib@openssh.com
  debug1: kex: client->server aes128-ctr umac-64@openssh.com zlib@openssh.com
  debug1: sending SSH2_MSG_KEX_ECDH_INIT
  debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

or

  ...
  debug1: Enabling compatibility mode for protocol 2.0
  debug1: Local version string SSH-2.0-OpenSSH_6.7p1 Debian-6
  debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
  debug1: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000000
  debug1: SSH2_MSG_KEXINIT sent
  debug1: SSH2_MSG_KEXINIT received
  debug1: kex: server->client aes128-ctr umac-64@openssh.com none
  debug1: kex: client->server aes128-ctr umac-64@openssh.com none
  debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<3072<8192) sent
  debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP

Then after some time the remote sshd closes the connection.

From a different VM, which is running Stretch, I can connect to these
hosts without trouble.  The debug log starts the same in both cases.  In
the working case, after the "expecting...REPLY" line I see a line with
the server host key (and more stuff after that).

Any suggestions for what to do next?

thanks,
mike