I was trying to start a game of Co-Op with a good friend of mine. I started the game and invited him, he joined and then we got the usual loading screen. Then I got the screen that says; waiting for your partner. While my friend was still loading, the game suddenly went back to the Main Menu saying my partner has disconnected. But my friend was still on his loading screen.
We tried this several times, but came up in the same situation. Then we turned it around. He started the game and invited me. The roles were now reversed. He got the screen; waiting for your partner and I kept staring at my loading screen which would not progress past the first circle in the lower right corner. Then after roughly 2 minutes my friend said I was 'disconnected' from the game.
Windows Explorer says that the Portal 2 client is still going, so it didn't crash. We also tried multiple times rebooting both our systems and rebooting the Steam client.
Does anybody else has this issue or knows how to fix it?