I had this problem and solved it easily
Just delete the files in C/…/Documents/FSX or C/…/AppData/Roaming/Microsoft/FSX or both .
Open then FSX and wait a little bit till it generates new files and bom , problem solved without losing any planes or sceneries that existed before the error , you only lose your setting configurations and your logbook and history of missions done .
( Maybe in Documents/FSX there's only a few files you could just delete and have your settings and logbook also saved after the error is resolved , but I don't know which ones so I deleted all )
