Authentication Endpoint does not send JSON as Response

We are using the API to retrieve certain information on courses, skill plans and pathways. Until last week everything worked fine, but all of a sudden the authentication endpoint sends a html-response instead of JSON.

If post the request like I used to, I get a HTTP status 200 and if I add a scope that is not allowed for my API-key I get a status 400 with "invalid credentials". So it seems to me the API-key and the authentication are fine. However the response is not, as the content-type of the response is now "text/html; charset=UTF-8".

Am I doing something wrong or is there a problem with the API?