The HTTP Error 403.6 indicates an IP address being blocked:
HTTP Error 403.6 - Forbidden: IP address of the client has been rejected.
Internet Information Services (IIS)
In order to access Sitefinity backend and SitefinityWCF RESTFul services to operate correctly the /Sitefinity folder should be freely accessible. Can you please enable service trace logging for your application following this KB article
, and send us back the generated logs after reproducing the issue several times, so we can have some updated information on the problem.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items