Private key invalid after restore from backup

so, we finally got the green light to upgrade our Nextcloud 15 instance.
Unfortunatly, this didnt go well and i decided to restore a backup from before the upgrade.

However, this lead to the result that users receive a “invalid private key” when they login, and nobody can access their files.

i am now decrypting all files through the commandline, a search on the forum led me to those commands, but does anyone have any idea how this can happen? I will disable NC’s encryption completely, ive never been a huge fan of it (too much troubles for its worth) but i would still like to have some understanding of how this came to be?

I would assume, if i restore a full backup from say, last night 0:00, all would be as it was at 0:00, and i would not have to be comfronted with this strange error?