I think this is a rodent ulcer - it's caused by an allergic reaction to something. I have a Burmese girl who gets this occasionally, she also gets sores / scabs around her neck. I don't know what causes the problem, it is probably a food ingredient. Your vet will probably suggest a steroid injection.

the ulcers don't usually last very long and they don't seem to bother the cat much at all.

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Problem has already been solved.. Thanks though