I am using the Standalone Report Designer to connect to the Report Server for .NET. However, in the .NET version, I see an error "The JSON value could not be converted to System.Data.DataTable.") when opening the query designer or when attempting to use the preview functionality at the last step of the SqlDataSource wizard.

In the .NET Framework version of the Standalone Report Designer the wizard itself works as expected, but the Data Explorer does not show the available fields and I have to enter them manually.