X forwarding timing out

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Got a very strange error keeping me awake all night long. On one of my
clients the X forwarding-session dies after successful authentication:

debug1: Authentication succeeded (keyboard-interactive).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Sending environment.
debug1: Sending env LANG = fr_CH.UTF-8
debug1: Sending command: xterm
debug1: client_input_channel_open: ctype x11 rchan 3 win 65536 max 16384
debug1: client_request_x11: request from 47103
debug1: channel 1: new [x11]
debug1: confirm x11
debug1: channel 0: free: client-session, nchannels 2
debug1: channel 1: free: x11, nchannels 1
Read from remote host foobar.example.com: Connection timed out
debug1: Transferred: stdin 0, stdout 0, stderr 69 bytes in 966.5 seconds
debug1: Bytes per second: stdin 0.0, stdout 0.0, stderr 0.1
debug1: Exit status -1
Server Side:
Oct 21 10:50:57 foobar sshd[5000]: debug1: channel 3: new [X11 connection
from port 49489]
Oct 21 10:50:57 foobar sshd[5000]: debug2: channel 3: open confirm rwindow
131072 rmax 16384

The "open confirm" stays for approx. 5 minutes, then the connection times
Tcpdump on the two firewalls in between shows an established tcp connection.
Other clients in the same subnet are connecting without problems AND the
problematic client is able to connect (and X-Forward) to other servers.

Any help or suggestions would be very much appreciated.


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