Heya Effjottess,
There could be a number of factors, the most common one for me is a missing car from the opposition...
But all of my Championships are made using GTL and GTL added cars, so in my case it's most likely me overlooking something... (I may have changed a HDC and misworded the HDC pointer in the car file, something like that woud cause a crash at load)
: I would check all cars on all of the circuits in 'Private test' to ensure no crashes.