F12 should open the browser developer tools, one panel will be the network requests.

It could be the accepts header then… check if the request includes accetps: application/json

In my experience that is almost always the server returning an html error page.

Start with inspecting that actual response the first character is probably <. The rest of it is likely to be a “not found” or “internal server error” (being the most common) page.

Then look at logs…