Oauth2 configuration problem

I’m trying to configure oauth2 for pingfederate and am stuck.

Here’s what the callback looks like:

Started GET "/auth/oauth2_basic/callback?code=ezrfkur2ftxlw-lfyZqKXGfe-_l1ATjzdapYIAAB&state=c1fccd278df1085b067614646cc51579e28f290e3cd93dd7" for at 2018-04-12 15:30:04 +0000

and then I get this error:

The client MUST NOT use more than one authentication method in each request.

@sam, is there something obvious that this error suggests?

Found this post when searching for a solution to the same issue.
I have found the solution and described it here: OAuth2 Basic Support