If only one of the mice in the same cage has scabs, it’s one of 3 things – the scabs are from the 2 mice fighting, playing, fighting or grooming each other – or the scabs are from too much protein in the food. Some rats can handle it, others have a rash that causes them to itch and scab. If both rats had it then I’d say it was mites – but not when only one has it. What do you feed them? The protein level should be around 14%. Some lab blocks made by Purina and packaged as Mazuri have 24% – too high to be a staple of a rat’s diet. The same goes for some dog foods. If you supplement the daily diet with plenty of fresh fish, poultry or meat, this can also cause this.
The only other possibility would be an allergic reaction to the bedding. I had a mouse that couldn’t handle poplar when using it for bedding and I had to buy him the very expensive Sanichip which was also made from poplar but he had no reaction or rash.