Cloud save corruption. Steam has serious issues with file corruption for those who use their cloud saves. It might take a few seconds for the cloud save to finish and when the game closes out it corrupts the cloud save files.

Always disable steam cloud save on every game. I recommend deactivating steam cloud alltogether. Problem solved.

No problems here and I'm running without cloud saves. If you have scheduled backups, your game data should be included in them, If I was IG, I'd remove the strong admonition to use the Steam cloud that now appears on the startup screen until Steam gets its act together.

Instead of just creating a single .old file that gets wiped out pretty quick, Valheim should accumulate numbered backups until you decide to delete them. Feature creep: backup to a specified path. Automatically delete backups older than <timestamp>.

And let's have event-driven saves instead of the time based save. I got into a rhythm, today, time save and garbage-collection hitting one after the other.