I think, I experienced this bug as well (same kind of backtrace). In my case this happened, when installing nxproxy and x2goclient in a non standard place which is not on the path.
The client tries to start nxproxy which fails. x2goclient never checks, if the start suceeded or not (see x2goclient-4.0.0.1/onmainwindow.cpp, lines: 3135, 5065).
Workaround: add nxproxy to $PATH.