Thanks for the suggestion Igor, but I haven't tried it, because I decided to switch to Bookworm before I read it, and then was too late. When I installed the Armbian_23.11.3_Rpi5b_bookworm_current_6.1.68_cinnamon_desktop.img I did not get the complaint about the acceleration. I installed the MESA v23.2, it went without a
glitch!
Unfortunately, a vcgencmd cmd fails the same way as in Jammy (i.e. VCHI initialization failed). I depend on vcgencmd to measure CPU temperature for cooling fan control, so I'll have to switch to some other way for fan control.
Thanks for your help!