DotnetOpenAuth RefreshAuthorization blocked 403 Forbidden (12202) - authentication

Question

I've a DotnetOpenAuth authorization server which works great on my localhost. However after publishing it my refresh access token request is blocked.
The request for a accesstoken, with success
POST https://myurl/identity/oauth/token HTTP/1.1
Authorization: Basic dsjSDLFJKSKLJesww
Content-Type: application/x-www-form-urlencoded; charset=utf-8
User-Agent: DotNetOpenAuth.Core/4.2.1.13026
Host: myhost
Cache-Control: no-store,no-cache
Pragma: no-cache
Content-Length: 86
Expect: 100-continue
Connection: Keep-Alive
username=theusername&password=fancypassword&scope=somescope&grant_type=password
The refresh request:
POST https://myurl/identity/oauth/token HTTP/1.1
Authorization: Basic dsjSDLFJKSKLJesww
Content-Type: application/x-www-form-urlencoded; charset=utf-8
User-Agent: DotNetOpenAuth.Core/4.2.1.13026
Host: myhost
Cache-Control: no-store,no-cache
Pragma: no-cache
Content-Length: 272
Expect: 100-continue
refresh_token=_ttH%21IAAAAGiYhlufAaXURH5P2oDOnPYgJx7YhoR33isvZkPPvlyUgQAAAAHoBYyDMLhq1qwGHHH2uGrLoHZli77XHbCnSFJSKLFJ3kl2j3klj2kljKFSJKLSJKL#$k3ljfsklfjl2
And the response:
Technical Information (for support personnel)
Error Code: 403 Forbidden. The server denied the specified Uniform
Resource Locator (URL). Contact the server administrator. (12202)
Any help, guidelines, pointers in any direction, would be very much appriciated!
I changed the url/username/password/scope/base64/refreshtoken for this example.

Answer 1

Their seems to be a setting in the TMG Forefront - Authentication Delegation which blocked the request.
Method used by Forefront TMG to authenticate to the published Web
server:
No delegation, and the client cannot authenticate directly
No delegation, but client may authenticate directly
It was set to option 1 after changing it to 2 the request is no longer blocked!