While assigning crusaders to missions, they sometimes reset from one second to another, leaving all mission slots empty again (much as it would look like, when you deassigned the crusaders from their mission slots manually). Very annoying, but not a problem normally, because you can just assign them again.
Unfortunately, it happened to me last time that I started a mission, when the reset happenend in the exact same moment.
The mission is now in progress without any crusaders participating, and without a chance to fulfill.
I uploaded a picture to make it clearer:
http://de.tinypic.com/m/jfiu55/1

Best regards!

Hi Eike,

Thank you for the report and the screenshot.

I'm really sorry to hear you encountered this bug. We'll try to reproduce it and get a fix in, once we're back in the office.

Erika

Just had a similar bug in which the crusaders disappeared when i hit the start button - but the mission remained on the table without crusaders; on the main screen it showed the two crusaders i had tried to send on the mission as being "on a mision" with zero seconds until they returned. Back on the mission screen they were nowhere to be found, and the mission i sent them on was still there with empty slots waiting to be done.

Happily, after a quick reload of the game, i found said mission to be in progress with the assigned crusaders and the appropriate amount of time expired toward it's ending.

Maybe not exactly the same bug as mentioned by the others above - but it did seem to happen right when new missions were appearing (i'd just completed several).