Well, I used this method as a last resort not expecting it to work.
1. Back up your levels/maps in C:/Program Files/The Game Creators/FPS Creator.
2. Go back to C:/Program Files and delete the WHOLE game creator file.
3. Find the installer/disc you used, and open the setup. When prompted, click repair and let that finish.
4. After repair has finished, go back to C:/Program Files/The Game Creators and copy the FPS Creator folder to my documents.
5. From now on, to open FPS Creator go to My Documents/FPS Creator/FPSCreator.exe
This fixed it for me. Whenever I open the one in C:/Program Files/The Game Creators it gives me that error. Whenever I open the one in my documents it works fine. Dont forget to place your backed up levels into the my documents FPS Creator, and always save your levels into the my docuements version.
Tell me if this worked for you