Alright, now it came back to me; had confused the situation with another issue I recently had with my Xperia tablet. So let me recap the current situation with my Z1c:
Device is still running the January build; updater keeps telling me that there aren't any updates, no matter how often I press "refresh"
flashing one of the recent updates (February or March) from twrp both result in this behaviour:
- after installing in twrp choose "boot to system"
- LOS boot animation appears and runs for an exceptionally long time (feels like 3 - 5 minutes)
- then still in the animation screen goes black, then SONY logo appears
- no boot animation now, but reboot to TWRP
- from the working January build I created a simple backup of just the boot partition; after trying to flash a newer build, and then back in twrp I restore just that boot image ==> does not help
- before flashing the update build uninstall Magisk (V 21.4) from TWRP, then flash the update ==> down't help either
Apart from doing a complete wipe then trying a new setup: any idea what I could try to solve this?