I have the same problem.  I had it a long time ago and did find a fix for it on the internet. Now I have re-installed FSX and all the addons, the problem has re-occured.

From what I can remember, it has something to do with the location of the flightplan folder and the location of the main FSX folder, but I just can't remember the "fix".

If anyone has a clue, I'd be most grateful.

Cheers

Glendower