The output of:
dpkg -l | grep -iE '(x2go|nx)'?
... Was too large to post to the list so I've thrown it up on my ftp
server, you can retrieve it from ftp.eskimo.com, username: anonymous, password: (your e-mail address). It is x2go-mint.txt.
Naturally without the ? character at the end, but in any case, you're on the release version.
Hmm, that error message is weird. It indicates that the remote proxy sent garbage, while the other side expected a version number.
What's the client OS? What packages are used there?
Mihai