I queued for CC on my cleric (25) through the LFG tool. The invitation popped up and I was teleported to the dungeon. However, I could not see party members, because the group was full and I was the 6th player. The game kept spamming me with "[Error] Error joining - The party is already full." I left the dungeon through the entrance, but still kept getting that message and had to reconnect to fix the problem.
Edit: Actually, after reconnecting to the game, I STILL get spammed with the above error message. I have no right-click option to leave the party (I also see no party members). And I cannot queue for the dungeon again. It doesn't show up under "K", at all.
Trying to join the queue for another dungeon/PvP fails because, "Cannot join a queue while your queue instance is still active."
It fixed itself after a few hours. I had logged back on the character an hour later and the problem was still there. Logged out, hopped on another character and tried later in the day.
Comments
Update: I entered another area, changed instance # and then pressed "Return to Instance" and I was removed from "active queue"
Sounds like you found a better fix, though!