When you try to run a report (or) Generate a model from an existing Data Source (or) Open a Model using Report Builder from the Report Server, you get the following error,
An attempt has been made to use a data extension 'SQL' that is not registered for this report server.
One possible cause of this issue is that the registered SQL extension in your report server config file would have got modified / removed.
Open RsReportServer.config file located under <installation path of reporting service> / reportserver folder.
Locate the element,
And make sure it is there as below,
<Extension Name="SQL" Type="Microsoft.ReportingServices.DataExtensions.SqlConnectionWrapper,
<Extension Name="SQL" Type="Microsoft.ReportingServices.SemanticQueryEngine.Sql.MSSQL.MSSqlSQCommand,