So after ~250h in game and 470+ days in the same (own) world without any save game issues, my progress was set beck fist time at all one week ago. I lost roughly 20h of gameplay.

As far as I overlooked that situation, this began to happen, after my wife installed and used her own steam account to play valheim on my pc. So after some test we recognized, that everytime she logged in (her steam account, her valheim version), my newly reached progress was set back again to that same timeframe when I logged in with my steam account/valheim.
Another thing recognized was, my worlds and characters were visible in her account from start on, which is nice one hand, but shouldn´t be. Otherway round, her game settings were always set back after leaving her session.
And third, her game progress was always untouched, which may be because she played in a hosted game, so "her" world ist not on my pc and therefore in a "safe space".

So, please devs, address this and come up with a solution for this problem. There has to be multiuser support and untouched user accounts even on one PC. Maybe this is a steam issue, which shouldn´t be either. As suggested by other users, a possibility to create dedicated save games would be so much help.