I am not an expert with Breeding, but they are on the young side and likely only laid egg(s) last year. I would back away from Breeding (separate them) and take a crash course in learning the 'huge' amount of knowledge that you need to know before starting. Plus the purchase of all the equipment, etc...
If you have not, read back into Past Threads in the Breeders Forum!
The behavior could be driven by any number of things. Protecting her nest box being a big one. As stated, pull the box and drop the expectations way down for a couple of years. If you really want to Breed, learn, learn and learn more! If you can, go work for a quality Breeder for the next couple of years (free of charge).