Ok I still frequently run into the corrupted EFS partition issue. I have no idea what actually is causing it. However, it can be fixed using the method from here: How to Fix Empty IMEI, Corrupted EFS, No SIM, Unknown Baseband and WiFi/Bluetooth issues after restoring a TWRP Backup
While that works reliably it is quite cumbersome to type in the erase commands in the tiny little terminal window. Therfore I have just written the commands in a little script saved on the sdcard, so that in the future I just need to run the script.
Here is how this can be done:
in the terminal window type:
echo '#!/system/bin/sh' > /sdcard/imeifix.sh
echo 'dd if=/dev/zero of=/dev/block/bootdevice/by-name/modemst1' >> /sdcard/imeifix.sh
echo 'dd if=/dev/zero of=/dev/block/bootdevice/by-name/modemst2' >> /sdcard/imeifix.sh
chmod +x /sdcard/imeifix.sh
now you can always run the script from the terminal:
/sdcard/imeifix.sh
Hope this helps others with the same problem.