Some people have suggested that that turning off the steam cloud saves for Valheim could help, but I read a few posts where people said they still have had the issue even with it disabled.

I don't know if it's a multiplayer thing, related to other servers, something to do with multiple users on one system, or something else. I play one one server with one character and have one solo game on the go with another character. I'm using the steam cloud but no one else uses Steam on my system. Everything has been fine for me so far. Maybe add details on the bug forum if you've been sharing the computer with someone else who has a different Steam login, or if you use the family share thing, or if you're using a bunch of mods.