Hi there,
Same problem here but after try to start offline ,Avorion won't start anymore about SteamAPI error.
I hate such errors which only about copyprotection (steam)...
I will keep trying ...
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Okay, now i got rid of the steamAPI error (Installed Avorion beta + switching to proton hotfix) and the game starts again.
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Now i've Avorion Beta installed and if i start the server from shell i can connect to it in the multiplayer menu.