Update. My friend can enter the game while i am stuck in the loading screen. For him i was invisible but my weapon was floating in my base. So i went back to settings and turned on friendly fire. When i loaded again he killed me and i actually respawned and left the loading screen. The only weird thing was that it did not give me the option to spawn at my backpack, instead gave me 2 times the option to spawn at my bed. But at least i spawned and was back in the game.
I then quitted the game to check if i can also play alone again but again was stuck so we repeated to process, then removed all beds, placed new ones, quit and reload again and now it seems to work. What a messed up @%$#. What a shame to release a product in this state. No backup save files, no option to repair savegames. Maybe fun pimps should take a look at GROUNDED on how to do this properly. In total i payed 70 € for a game that does not work properly for years. Good job, not.