Please switch to loglevel=debug in /etc/x2go/x2goserver.conf and see what syslog brings then.
If you have not updated x2goserver.conf yet then place this stanza into the file
[log] # possible levels are: emerg, alert, crit, err, warning, notice, info, debug loglevel=debug
Thanks for the tip, unfortunately /var/log/debug didn't give me any additional clues to what the problem was.
After several hours of troubleshooting I finally gave up, removed and reinstalled Gnome. So far it's looking good, I can start sessions and reconnect to them. I'm using nightly builds, i e the new libxcomp3, and it works. I guess the problem was in Gnome/dbus alone or some combination of them and x2go.
Cheers, Daniel