Hi and thank you for your reply =)
raoul We typically see this sort of behaviour when your computer's graphics card doesn't support OpenGL, or you ran an upgrade on your computer but didn't restart yet.
Well, I didn't verify it yet, but I'm quite sure, that my 2012 Macmini with an actual Ubuntu does support OpenGL and I know for a fact, that I restarted it serveral times, especially after update, upgrade, dist-upgrade...
Since my technican in charge wants to get his Macbook (with MacOS) back, I solved my problem by installing (L)Ubuntu 18.04.5 as a second OS and after manually installing all dependecies OpenLP 2.9 worked fine. Next step is testing it under real circumstances in church. I'm quite sure, this setup will do the job... maybe I will try to Update OpenLP to v3, but as long as it works - never touch a runnig system ;-)