...and my (probably) final report to this mailing list:
The most recent crashes/hangs I experienced have no relation to Debian
release or Linux kernel, but most likely a hardware issue.
I initially experienced a crash similar to what the original report in
this bug-thread showed. Suspecting that the old machine was dying of old
age, I replaced it with a new machine - and got a similar crash with
that one as well.
Later on - after having a newer kernel installed that probably solved
the original bug - I got a few times a crash/hanging system without any
kernel messages and suspected this to be caused by the same bug.
This was likely a wrong assumption: in the past few days I've ran
Memtest86-pro for two days straight - and the system is hanging now
after 39hours testing. Since this Memtest86 is started from USB, it
means that the OS is not involved.
So, on to hardware debugging. I'm OK with closing this bug.
Matthijs