(explicitly CC:ing Anders who may give feedback on this)
I can connect to different servers, work with it, but as soon as I disconnect a session, the client won't access any key inputs. I can still click with the mouse and get reactions like opening the settings dialog, I can even enter text in the settings dialog, but I cannot log in anymore as I cannot enter a password (or change my username).Sound like this issue has been reported before: #139. Thus, merging these bug reports with one another.
This happens on different Macs, all using the latest Version of OS X 10.8 (one tested with 10.8.3).
The current problem about the Mac OS version of X2Go Client is that the upstream developers (Alex and Mike, mainly) do not have a Mac hardware at hand to get issues like this one fixed.
If someone can permanently provide us with hardware (or contract us so that we can buy some Mac OS X capable device), it will make it much more likely that Mac OS issues get fixed in a much quicker way.