After some experimentation, I managed to get a URL similar to the one you show (starting with "?response_type=..."), which isn't a valid URL. It should look more like this (i.e. starting with "https://...") ...
.
I'm not sure yet why the bad URL response isn't accompanied by a status code; apparently the back end is taking a "you asked for it, you got it" approach.
In my case, I was playing with the GMAIL authentication and the
OAUTH2.BP sample program. The issue (for me) came down to the fact that my client credentials did not include the necessary name:value pairs that the sample program was looking to extract from the json file. (The issue was that the Google API console had changed the names used in their json file.) But ultimately it's a matter of making sure you actually have values for all of the OAUTH2 parameters required for that API (client id, secret, port, authorization endpoint, token endpoint, challenge and scope).
That's probably not enough to solve your issue, but hopefully helps you focus on adjusting those parameters until you get a URL response that looks like an actual URL. But if that's not getting you anywhere, feel free to send me the details needed so I can try to access the API from here.