If I understood correctly, you use Telerik Report Server
Does the error occur in the integrated viewer of the Report Server Manager
(while attempting to preview reports), or are you using a custom Web application that utilizes the Report Server to deliver reports?
Generally, the error message indicates that the Report Server/REST Service supposed to deliver reports to the Html5 Report Viewer is not working, hence the client (viewer) cannot be registered with the service.
Test re-starting the Report Server from the IIS.
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