I've never observed that issue, but if it started working again after
you ran
sudo apt install --reinstall ubuntu-gnome-desktop
then my next question would be whether running vglserver_config again
after that caused the problem to return.
In general, local login abilities will be limited once the VirtualGL GLX
back end is configured. Wayland will be disabled, as you already know,
but also, you won't be able to log in locally while anyone on the system
is using VirtualGL, or it will cause any applications running with
VirtualGL to freeze until you log out. VirtualGL 3.0 includes an EGL
back end that avoids these issues by eliminating the need for a 3D X
server entirely, but the EGL back end doesn't yet support all of the
features that the GLX back end supports (refer to
https://github.com/VirtualGL/virtualgl/issues/134 and
https://github.com/VirtualGL/virtualgl/issues/136), and it still has a
few bugs that I'm working through. Apart from those bugs, the EGL back
end should be suitable for most OpenGL applications-- especially
scientific applications (because those applications don't typically do
stupid things with GLX, whereas commercial applications often do.) You
can try it out in the dev/3.0 evolving pre-release build, if you want:
https://virtualgl.org/DeveloperInfo/PreReleases.
DRC