It was a memory issue, but watchdog did not solve it.
my swap space of 100MB ran full within an hour or so, so i made it bigger to 150MB.
The MM ran for a week, and it froze again…
Made the swap space 2 GB ( twice the memory size of my Pi 3B), and it did not crash or freeze since i made that adjustment.
swapfile is around 220 MB at this moment, so 2 GB is a little overkill i guess
The pi does a scheduled reboot every morning. (As it has always done)