Re: x11base change
For sure I messed something up, as now, with a 6-STABLE freshly installed, I cannot reproduce the error.
Anyway, the case with the "Failed to initialize GLX extension" error was not the problematical one, this was only to show that with the base nv driver the segmentation fault didn't occur. I suspect a more generic, OS corruption just arising there as the system is usually under no load. But this is history now.
Thanks to all of you who have tried to help.