Whilst it would be good practice for the Employer to explain the reason in more detail than just quoting the term “any other reason”, strictly the contract doesn’t require this. Technically the Employer might be in breach of clause 10.1 i.e. absence of a detailed explanation of the reason is a failure to co-operate effectively with the Contractor, however this argument doesn’t really take you anywhere from a commercial point of view.
The only other argument I could think of would be to claim the Employer’s termination isn’t in accordance with the contract (but as already discussed, is it really?) and that it amounts to repudiation. Again though this doesn’t take you to a better place commercially and the contract won’t become un-terminated.
Remember that because the Employer chose reason 5 you will recover “5% of any excess of a forecast of the amount due …” which is intended to be a sweetener for the inconvenience you’ve been caused as you weren’t in breach of contract.