I never experienced that problem before. Maybe the OS just got a little smarter now:
kernel: [ 6072.179381] Critical temperature reached (91 C), shutting down.
I opened the notebook (if I only had known, how easy it is to open this one), cleaned the CPU fan (wow, what a lot of dust!), …, finally I left the bottom cover off, and put the notebook and 2 dishes instead, in order to help with air circulation and heat transmission.
The CPU fan does no longer run smoothly, ordered a new one. Costs me like 90 Euro. Does not look that expensive, but then – what are the alternatives?
Managed to configure gkrellm, so that it shows temperatures. So I know at least, when it gets critical resp. how far away from critical it is.