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Reporting Services (SSRS/MSRS) 2008 Error: Set used with the complement operator must have all members from the same level

When you use the Not In operator in a SSRS 2008 MDX query filter to exclude a named set, it uses a the complement operator in the constructed MDX. This is fine as long as "all members [are] from the same level." Since you got this error, they are not You can get around this by using the Except() MDX function instead of letting SSRS use the Complement operator

In the ReportServerService log, you'll see something like this:
Microsoft.AnalysisServices.AdomdClient.AdomdErrorResponseException: Query (..., ...) Set used with the complement operator must have all members from the same level.

Original filter
Dimension: Time
Hierarchy: Calendar Date
Operator: Not In
Filter Expression: [Today]

New filter
Dimension: Time
Hierarchy: Calendar Date
Operator: MDX
Filter Expression: Except([Time].[Calendar Date].[Calendar Date].MEMBERS, [Today])