I assume that the issue occurs while adding new report in the Standalone designer.
The error indicates that there is no connection to the Report Server, or user credentials are not correct.
Check if the Report Server is started in the IIS.
to check the exact requests to/responses from the Report Server for hints on what might have went wrong.
If the problem persists I suggest to open a support ticket and send us the SAZ
file generated by Fiddler while experiencing the issue.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items