- Verified my cache.
- Restart game
- restart steam
- restart pc
Well, only thing left to do is to re-install the game I suppose. Although verifying the game cache should have done the trick. Very odd thing indeed.
If you accept the invite from within the multiplayer menu (after you get the message about creating multiplayer lobbies)? Please try this before a re-install.
We kept trying it and eventually I just got in the game, took about ten restarts. =-=
Probably could have checked your friends version, or have them restart their game. See if that would have worked.
I got this right after I reinstalled. Derp me then realized I should probably click on “Multiplayer” to download stuff that you can only download in the client, and THEN successfully joined the game.