Not sure what you did there, but I would post this (maybe with a screenshot) on the official Valheim Bugtracker.

https://valheimbugs.featureupvote.com/

Have you been running mods and used some feature from them that allowed you to build floating castles or something?

No, no mods that do things like make stuff fly. I did use the unlimited heightmap mod though.

Now, my issue is how to get my old backup save to work again. The FAQ and other topics I have found so far are very unclear on this. There is supposed to be a backup suffix somewhere to my world save, but nobody specifies where that would be, what I need to do, where I need to copy, etc. I am incredibly busy and gaming nights usually are scarce. I don't have time for this ♥♥♥♥♥♥♥♥.