Request Access Token failing..

[wbuist]

I'm seeking to access Nimble (CRM)'s API which is a pretty standard OAuth2 process as far as I can tell.

When I request the Authorisation that works fine and I get a login screen and then redirected back to my redirectURI with an appropriate code.

In the next step, requesting the token, I get an error which says: An Authentication object was not found in the SecurityContext

Any idea's? Sorry if this is obvious I'm not well versed in Oauth yet.