- Your final track sector isn't overlaying into the first track sector, I think this is what is causing the crash. Look at how the original GP4 tracks are done to see what I mean.
- The pitlane is waaaaaaay too sudden off the track, one of the ends of the pitlane isn't even layed fully into the track. This could also be causing the crash.
- Don't forget matching the heights of these things, the places where the track connects itself, and where the pitlane connects the track. In my first tracks I always did these parts completely flat just to avoid these headaches.
Now more of a personal opinion, there is definitely some very interesting parts in the layout of your track, but it could maybe use a longer straight here and there, also you could pay a little more attention to how the kerbs are done.