'Error Loading Mission'. What is this,please?

[JG52Uther]

Not seen that one! I did have a very weird problem several months ago where I couldn't fly the Hurri Rotol at all, and any mission with that plane in caused the game to hang at 95%.
I tried all the reinstall/verify stuff to no avail.What fixed it in the end was turning off steam 'cloud' and going through the steps again!
Not saying this is the fix for you, but it might be worth a try if nobody comes up with the answer.