If you click "Find Match" in the lobby as it becomes available when you create a lobby, it prevents you from clicking either the "Find Match" or the "X" button, changing the lobby type doesn't fix it, but relogging does.
1. Create a match.
2. Hit "Find Match" as soon as possible.
3. Get locked out of starting a game.
Edit: I just logged in and went into a party lobby, and I didn't press anything, but I couldn't click the X or Find Match, so maybe it's just the first lobby I create that I am unable to find a match in