This endpoint specifically, though the other endpoint you referenced doesn't work either. However, I have used this same Api Token to hit every email end point, to create tickets, and to associate emails with tickets (such as your API allows), so I'm reasonably sure it's not the request.
Apologies on the delay. 100% my mistake. Verified the issue and the engineering team is working on this now. I will report back (I promise) when there is a resolution. Thanks for the patience 😀