Steam cloud saves often cause conflicts with the world saved on your computer, it is just not a reliable process unfortunately :( they can easily get de-synced. Which can cause the world save on your computer to get messed up if the old cloud save takes priority.

I would turn it off for VH if I were you so that you can be confident the world save on your computer is the controller for your progress. I'm sorry for your lost efforts :(

Edit: It's a good idea to turn off the Steam save for most games tbh. Especially games like this where you're putting a lot of time into creating custom bases.

It sounds like it could be an issue with Steam Cloud Saves, do you have that enabled on your Valheim install?

Always, but I have never deleted Valheim in the first place.
Something happened in the meantime, maybe after some patch or w/e..

My character has the same gear as it should, just the game is "restarted" before the third boss.
My map is as it should be, with all "marks" written on it.
Not sure what happened for a save file to restart after the second boss.
I didn't check did my first base got removed, i just got disappointing because my base that I built for more than 50 hours got deleted...