Greetings.
I tried building x2goclient, x2goserver, the NX libraries, etc. from the respective Git heads, some of which were a few commits ahead of what I had been using. That didn't fix the problem. I then switched from the packages I built myself to the nightly builds at <http://packages.x2go.org/opensuse/tumbleweed/heuler/>. With the latter the problem is not reproducible, which is strange, because AFAIK these packages should have been built from the same sources.
Unless anyone else can reproduce the problem, maybe we can ignore this issue for now, and I'll just continue using the builds from the heuler repository. (Though note that this repository lacks packages for the latest release of openSUSE Leap -- see Bug 1491 at <https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=1491>.)
Regards, Tristan