Raspbian: After update from buster to bookworm, X11Forwarding in ssh connection stopped working

["B.M." <b-misc@gmx.ch>]

Dear all,

I just dist-upgraded my Raspberry Pi from buster to bookworm, and while

ssh -Y...

worked like a charm in before the update and I could start any X11 program
over ssh, it doesn't work anymore since then. Executing

ssh -Y -C -l myUser otherHostname.local -v

I get

...
debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Sending environment.
debug1: channel 0: setting env LANG = "en_US.UTF-8"
debug1: channel 0: setting env LC_MONETARY = "de_CH.UTF-8"
debug1: channel 0: setting env LC_MEASUREMENT = "de_CH.UTF-8"
debug1: channel 0: setting env LC_TIME = "de_CH.UTF-8"
debug1: channel 0: setting env LC_ALL = ""
debug1: channel 0: setting env LC_COLLATE = "C"
debug1: channel 0: setting env LC_NUMERIC = "de_CH.UTF-8"
X11 forwarding request failed on channel 0
...

From /etc/ssh/sshd_config on the server:

AddressFamily inet
X11Forwarding yes
X11UseLocalhost no

Interestingly, when connecting for the first time I got a warning:
WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!
and I did just
ssh-keygen -f "/home/xxx/.ssh/known_hosts" -R "otherHostname"
which I did.

xauth is installed on the server.

What can be the reason, that I cannot use X11 forwarding anymore?

Thank you.

Best,
Bernd