The screen shot attached shows the GSX Menu showing 'File Option' instead of 'Start Boarding' etc....
That's why I asked if you were sure about the "filename", since I couldn't see how you could possibly have it named that way. And in fact, they weren't, it's "File option" + a number, not "filename".
The problem usually means an incomplete installation, but since you said it only happens in some flights, that cannot be, unless you have ran the FSDT Update after this happened, in that case it might be an antivirus problem that has been temporarily fixed by the updater so, the first thing to try would be adding the whole Addon Manager folder to the antivirus Exclusions.