@Robert Guetzkow (rjg): Due to other dependencies, I couldn't install the previous stable version. I just add that I have upgraded the system, the kernel and upgraded the version to Mesa 20.3.0-devel (git-6668d7a 2020-08-16 bionic-oibaf-ppa). Unfortunately, I still have same problem. I'm beginning to suspect that it may have something to do with the graphics card (Asus R9280X-DC2T-3GD5), and not only on Linux. I checked on the old Windows 7 system and the lower part of the cube is displayed, but strange artifacts appear. Before that, I didn't see any problems with 3D games. In 2.80 I can see the model correctly. Strange