Getting this error in the browser when trying to preview a report:

Error registering the viewer with the service.
An error has occurred.
Incorrect value (null) deserialized. Make sure you are using CacheStorage inside single-instance application deployment only.

A possible reason is the cache for the Report Server used for previewing reports has become corrupted. A workaround is explained in the Telerik.Reporting.Cache.CacheStorage.AddInSet NullReferenceException KB article.

It will be very convenient for the user to be able to clear the cache directly from the Report Server Manager UI.