Dear Sandro, dear all,
I have now also installed GLFW, on my Ubuntu 18.04 I used:
sudo apt-get install libglfw3-dev libglfw3
With this, finally the issues with the DebugGUI are solved and also the rest of the build went through smoothly, meaning I have a fully working O2 and QC again, the first time since before Christmas!
Thanks a lot for your help, maybe the line above could be added to the QC Quick Start Guide together with the info, that installing GLFW is obligatory now (at least for the moment).
Cheers,
Stefan