Thanks for the Answer.
After factory reset, the "modem GPIO with label modem is not available" is still there, however the device reboots. So this was actually not the issue. Also nothing in the system log, just
Tue Nov 15 13:10:20 2022 kern.info Request from SSH / Web UI,
Tue Nov 15 12:53:03 2022 daemon.info procd: - shutdown -
Then business as usual (no shutdown). Also nothing in dmesg.
I also had some issued because I used up most of the flash (partly my fault, partly the behaviour of the default backup).
*Finally*, the issue has to do with my script running out of /etc/rc.local
3400 root 1604 S /bin/sh /etc/config/autoatten/autoatten.sh
The script is only using snmpget and curl. If I kill 3400, the device reboots.
Is there a simple way to run this script from /etc/rc.local AND still have a working reboot funtion?