Black screen after updates

I'm seeing a black screen after reboot following most but not
all recent updates to bookworm on a Pi5. The strange thing is
that rebooting from a somewhat older Bookworm microSD seems to
permanently (until the next update!) fix the problem, allowing
normal boot from a USB hard disk.

Meanwhile, both Chromium and Firefox have quit playing videos,
remaining stuck at the loading screen but updating thumbnails
if the progress slider is advanced.

Anybody else seeing this?

Thanks for reading,

bob prohaska