I'm also having this issue. The error first occured when I was playing a game (match #3809688) with some friends in a party in which the server crashed mid-game (probably due to steam maintenance). A couple of us got back in the game and finished it but afterwards I couldn't queue up because I was apparently in a party, despite not being in a party.
So far I've tried the following with no success:
- Had a friend invite me to a party (couldnt because I was already in one)
- Invited a friend to a party (couldnt because I couldnt create a new party until I leave my current one)
- Forcefully change my region to another one using the "citadel_region_override" command
- Restarted the game
- Verified integrity of the game files
- Reinstalled the game
- Restarted my PC
- Uninstalled the game, deleted everything related to "citadel" and "project8" from my PC, and then reinstalled the game
Seems to have something to do with my account (76561198029215679), as opposed to the client.