@girish : SOLVED !
I traced it to (1) the backup in cloudron update failing, triggering update to fail, which was totally caused by (2) my ultra-minimal minibase not having sqlite3 in it and I forgot to add it manually as the packager responsibility when going ultra-minimal.
Interestingly I did not discover this in testing, because I was always ticking 'skip backup'.
So your approach in the other discussion is proven out !
Ultra-minimal can work but it seems not wise, because darned packagers cannot be trusted .
A slightly fatter base is worth it to avoid such mistakes. You win !
Will fatten my minibase while waiting for yours.
Sorry for taking up your time again.