Is it possible for a fox to breed with a rabbit?

It probably can't actually get into the hutch unless there is a hole/gap in the wire front or something, or if the catch on the door is loose.

However the fox doesn't actually need to get into the hutch to harm the rabbit, just the fact that the rabbit can see or smell (or even hear) the fox could cause it to become frightened or shocked. Rabbits can actually die from fright so be very careful to make sure the hutch is in an area where a fox can't access it. If you really care about keeping the rabbit from harm you could consider keeping your rabbit indoors, if not as a permanent "house rabbit" then you could at least keep it inside overnight.