set webcam as a global source instead of putting it separately in every scene. This keeps the drivers and your cam active (separately put sources restart everytime you change scene) and prevents these kind of issues happening.
So the problem is that your cam or it's drivers or both do not like restarting often in short time.
If that doesn't help could you post your log where you used that scene and got red input? (here is how-to:
viewtopic.php?f=5&t=97 )