I'm running into same issue. SharePoint 2010

when setting up "Manage Data Sources", getting following error on all reporting service reports..

Report Server has encountered a SharePoint error. ---> Microsoft.ReportingServices.Diagnostics.Utilities.SharePointException: Report Server...

=COUNT(Fields!product_key.Value)/COUNT(Fields! product_key.Value, "replace this with your dataset name")

this might help !!!