Can't find precise details. Sex Matters say she is suing the trust for discrimination, harassment and breach of her human rights.
From what I remember reading before, she is arguing that being forced to refer to someone by their gender identity, rather than their sex, conflicts with her religious belief that you cannot change sex. Religious belief is of course a protected characteristic under the Equality Act. I don't know if she also intends to claim that it conflicts with her gender-critical belief, which is also protected, of course.
The Human Rights Act protects freedom of thought and freedom to practise one's religion, so that's interesting too, because previous tribunal claims have, if I'm not mistaken, relied on the Equality Act rather than the HRA.
Harassment is fairly straightforward, I think, in that they clearly disciplined her and made life difficult for her purely because she referred to a man as "he".