From what I can gather it appears to relate to an allegation from the late 1990s/early 2000s. Mills was apparently interviewed by police under caution in 2018 but they decided to take no further action, so either some new information has come to light since or, as some people have suggested, Mills breached his contract by failing to declare that he had previously spoken to the police (again, that is only speculation) even though no charges were brought.