So I'm just running through your typical grindfest for the Kataclysm, when all of a sudden I'm greeted with the lovely 'An error has occured with your Knight; message whenever I attempted to:

1) Start the mission I was running over.
2) Start a completely different mission.
3) Access my Guild Hall.
4) Click 'Join' on anyone in my friends list.
5) Do anything that isn't 'Go to Haven'.

So, logic dictates that I listen to the prompt, and log out, then log back in. From the ready room, here is what one would usually do:

1) Go to Haven
2) Start a mission
3) Go to Guild Hall.

Guess what I cannot do? Go to the Guild Hall, or start a mission. What did relogging fix for me? Nothing. Once again I am greeted with a lovely error prompt. I've submitted a Bug Report in-game on this. Whatever this obscure bug may be, please fix it. RNG based events wear my patience thin enough as is.