The answer is above, but it seems that I cannot leave a comment or upvote the one above! I spent about 6 hours tackling exactly the same, changing versions of Jersey, method signatures etc, before eventually realising that it was only the Python side to blame after testing the API with Swagger (which I guess I should have done to start with!).

I really think that something stupid is going on on the Python side that should not have wasted neither mine nor somebody else's time!