Hi, thanks for the answer. Yeah would be better to give more Info in the first place.
So it worked at the beginning I could play with a friend in MP and also with Mods. After I register and connect my Steam Account with mod.io (also connect my Google account to it) to create my first mod for our own use (just edit some stuff for our play).
I got Multiplayer grayed out instantly from the start. And above was our try to join the lobby with Steam invite, but it failed. I have done several verifying the files (3 - 4 Times). Also, my friend has no Issu and it working by him.
It weirdly tells us that I am not logged in and also says that I don't accept the DSVGO and just what? Btw where is the check Box for it maybe it let me then play MP.

Connected with Steam Acc.

In English maybe it helps more.