No, it's not the temp sensor. I meanwhile checked the following

Start and kill the engine every 2 minutes until temp >80°C. No problems.

Drive a bit, park car, kill engine, restart: no problem

Restart after 15 min: doesn't start.

Replace temp sensor with 320 Ohm resistor (=80°C): doesn't start.

Wait a while until car starts, drive a bit, kill engine, switch on ignition every 15s to keep system fuel pressure up (fuel pump starts with ignition on for 2s) for 20min: doesn't start


Now, I have no idea left what to look for. It seems as if it would be a timing problem after the car ran for a while.