With vRA 7.5 (HF2) I have an API explorer bug: Workitem Service API produces wrong URLs resulting in 404 errors:
https://myvra/work-item-service/api/workitems
This should be https://myvra/workitem-service/api/workitems
"Try it out!" button gets 404, the produced curl commands are also wrong.
That's a pretty early hot fix, though. Have you tried later ones? Some significant things are fixed, and they're cumulative.
Checked another instance with HF5 applied, same problem.
Well at least you know the right API to use now, just skip the API explorer