I used to get illegal operations when trying to save a default startup mode with any airplane (3rd party or default) and after a long trip ..save a startup at my arrival point so the next time it will start FS2000 there. Saving a default situation would always bomb me out of FS2000, with a variety of illegal messages next time I start up FS2000. This was always related to using 3rd party aircraft but then it started with the default airplanes as well. It would seem once this pattern of "doom" occurs, it gets worse and soon FS2000 would be rendered useless. Many sleepless nights were caused by these infuriating problems - threatening my FS2000 happiness forever.
THE FOLLOWING ENSURES YOU'LL NEVER HAVE THEM AGAIN....
YOU WILL NOT HAVE TO REINSTALL FS2000 AGAIN! I used to, thinking FS was getting damaged...but for two months now I can confirm this is the solution! After so many fresh installs, flying for hours on default only aircraft and then going over to add-on aircraft, I found the key for success!
If you ever had saved situations after using the GPS to navigate, this is where the problem has been all this time! If you don't use the GPS, then you probably have not gotten the illegal operations.
SOLUTION
After EVERY flight where you save a mode and leave FS2000, IF you had used the GPS in a direct-to function or a flight route...do the following:
[GPS_Engine] DirectTo=True
change the True to False
DirectTo=False
That's It!
Now, save and exit. You will be able to get right in to FS2000 using your "corrupt" startup situation, where before you would have bombed out.
Do this after every flight where the GPS was used, and you always get into FS2000 with any aircraft, anywhere, anytime with any panel etc. etc.
This is the cause of 99% of ILLEGAL OPERATIONS for all flightsimmers. I have verified this with many others over a two month trial period. It works. Please let me know if this helps you too!