Hi Mth,
Indeed adding the line:
AvailableSessionTypes unix-remote,unix-console,unix-default,unix-application,shadow,unix-xsession-default,unix-gnome,unix-xdm,windows,physical-desktop
to my server.cfg file solved my issue
(for some reason there was no such line in the server.cfg file, but only in node.cfg)
Thanks a lot for your helpful support.
Have a great day
Simone