Hi Andre.
I know it's been a while since you replied to this bug.
However, the X2Go project and the Arctica Project have started doing major code cleanups to nx-libs on the new 3.6.x branch. Many of those code cleanups are big because they involve making nx-libs using the system version of X11-related libraries rather than nx-libs bundling its own (horribly outdated) version of those libraries. Those changes will improve compatibility with some X11 apps, but not Qt5 apps.
After those code cleanups are complete, we will look into either rebasing nx-libs against the latest X.org, or backporting features such as newer RandR and XInput2 to nx-libs. In either case, Qt5 apps will become compatible.
There is no ETA though.
Code and issues are located here: https://github.com/ArcticaProject/nx-libs/tree/3.6.x Code is also located here: http://code.x2go.org/gitweb?p=nx-libs.git;a=shortlog;h=refs/heads/3.6.x
-Mike#2