All campaigns have been completed.

I had something similar happen to me once (but it was with the first part, and it seemed to be with the elven campaign, when I had to kill Nicholas again). After I completed it the first time, the game crashed, I don't know why, and the mage campaign became unavailable. Even autosaves didn't help (I just kept crashing after all of that). And when I decided to start the mission from the beginning, it wasn't very pleasant, nothing crashed anymore, and I was able to continue the quest calmly.