Hello Reinhold,
The order of the fields in the report definition is important. This error
message usually means they are not in the correct order in your request.
Also, consider using our C# libraries which will handle this and many other
minor details for you:
https://developers.google.com
I use AdHoc reporting with DownloadClientReport. I have this selection code
which is used in this example:
http://code.google.com/p/google-api-adwords-dotnet/source/browse/trunk/examples/adwords/csharp/v201206/Reporting/DownloadCriteriaReport.cs
I get this result:
ReportDownloadError.INV
