Although I can replay every other stage on all difficulties I cant get past stage 3 in nightmare in catacombs of Narbuk. I keep geting an error message every time I try to play stage 3.. My network is fine and all other aspects of the game are runnig fine right now. I checked all previous levels in nightmare that I have finished so far and I can replay them, I just cant play the campaign at Catacombs of Narbuk stage 3. The error message is the following: "Unable to load part of the resources. Check your internet connection and try again."

PROBLEM SOLVED: I guess the ongoing server maintenance at the backgroung was the issue. As soon as this was done I could go on with the campaign. Lesson learnt :)