Seems like the problem is that the "Level 1" scene was opened multiple times (a fairly rare occurrence, I struggled a bit to reproduce). Perhaps you triple clicked and the timing was just right to open the scene twice? Following that, the scene was unable to close properly.
This is fixed in the upcoming 4.0.2 release and in the latest subscribers-only builds (b10476). Fixed by not allowing a resource to start loading if it's already being loaded.