As I said, old site is running, I did a backup and put that onto a fresh install, restore failed. I modified the post noted as the culprit until restore on the fresh install worked. Only to see having the issue with Sitekiq again/still.
The old site runs postgres 13 as well (but goes back several years, so it most likely didn’t start with that version 
)
root@discourse-old:/var/discourse# ./launcher enter app
x86_64 arch detected.
root@discourse-app:/var/www/discourse# psql --version
psql (PostgreSQL) 13.5 (Debian 13.5-1.pgdg110+1)
So, according to this post’s closing comments, Discourse’s Database can get corrupted — and get fixed.
Tried with a new user, it get its registration email properly. But notifications on replies to its posts, no; Sidekiq errors out.
So, to me this means that Discourse is giving some fautly information to Sidekiq when it instructs it to send out Notifications (in contrast to registration mail). How to debug further?