Thanks for that reaction and that idea Alancito, after your questions I expected that kind of a suggestion. But I do not think that is the problem.
It is very unlikely that “some bits” would fall over during such an update, even the weaker battery would not cause a problem like that. Certainly not as the update was while the phone was on main power.
The update process has checks in itself that would warn if something would have gone wrong, and would certainly have warned if battery power was too low. On Reddit I also see that the 18.3 is a buggy release.
Anyway thanks very much for your support and thinking along with me in this problem.