Okay, now I'm completely confused.
I created a new mission in FRED and added the same 4 ships that were used in my other mission. This new mission works fined with no errors.
The old mission, however, still fails with the model error.
On the bright side, the error seems to be fixed, except for the problem of my other mission (which took hours to create) still won't work. But the error must not lie in the VP file as originally thought, otherwise this new mission would throw up an error too, right?
Well... back to the drawing board I suppose. I hate unsolved problems.
P.S. Yes, I am using the Ajax, Formidable, HHaifra, and Nimitz.