Okay, getting back to this. @sdetweil thanks for the tip, I bought a 32gb usb stick and made my Pi3B+ boot from it. Complete new setup of Raspbian Buster. Boots up fine.
Complete new setup of MM2 with your script, works fine.
Reinstalled modules and config, works fine.
Cooling with passive cooling case, Temp not higher than 55°C.
Setup a reboot cronjob every 6 hours.
Still, the system keeps freezing. Sometimes after minutes, sometimes after days.
Only other software installed is the lights-out client (which needs mono). (green-it-software.com/)
I have no idea on what to look for or what to do to fix this :-(