Greetings.
Tying up this thread, I've now reported this as a bug -- see <https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=1229>.
I've separately reported that the server doesn't produce useful feedback in this case (and I presume in other cases which fail validation for other reasons). That's <https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=1230>. I _think_ these are distinct issues, but feel free, of course, to merge them if I'm mistaken.
Best wishes,
Norman
-- Norman Gray : https://nxg.me.uk