Hi there Sliver,
Any chance that you can post the mission script in question? For some reason, Nexus doesn't like your missions ID tag; I'm not sure if we can help without the file to do some investigating with...
All the best,
EDIT:
Just done some experimenting, it appears that the highest mission number you can have is "299". So if you adjust the mission number to somewhere between "0" and "299", that should sort out your problem.