Hmmm, long paths are the only thing I can think or your FSX likes it's own specific PC format.
So try this.. Do you have LNM or Plan G?. Trying loading the 1st FP in there first , then as soon as it loads, export that same FP as FSX/P3D format then save it somewhere (not too deep).
Then try to load your saved FP in your FSX.
Norm