Crystal Reports 10 from C# vs2008: error "missing parameter values" but all parameters have values?

We have converted a project from using .NET 1.1 to .NET 3.5 (to run nicely on a new server) and all works fine except for the Crystal Reports.  
We were using CR 9 and now have to use CR10, since there seems to be no way to reference CR9 DLLs under this project now.
When a report is generated we are getting the error "missing parameter values".  We have changed the order and the manner in which we assign values.  We are also checking so we know that immediately before the call to generate the report, all parameters do have values.  We have run with our usual situation (some parameters have a null or blank value) and with every param having a non-null, non-blank value.  All get the same error.
We have tried a dummy report with no parameters and that works fine, so it does look like a parameters problem of some kind.

In brief, without boring with code, we:
- load the report
- set up the database connection
- set the parameters (values from web page)
- for debug purposes check if all the report's parameters have a value
- export to PDF

The full error text is
CrystalDecisions.CrystalReports.Engine.ParameterFieldCurrentValueException was caught
  Message="Missing parameter values."
  Source="CrystalDecisions.ReportAppServer.DataSetConversion"
  StackTrace:
       at CrystalDecisions.ReportAppServer.ConvertDotNetToErom.ThrowDotNetException(Exception e)
       at CrystalDecisions.ReportSource.EromReportSourceBase.ExportToStream(ExportRequestContext reqContext)
       at CrystalDecisions.CrystalReports.Engine.FormatEngine.ExportToStream(ExportRequestContext reqContext)
       at CrystalDecisions.CrystalReports.Engine.FormatEngine.Export(ExportRequestContext reqContext)
       at CrystalDecisions.CrystalReports.Engine.FormatEngine.Export()
       at CrystalDecisions.CrystalReports.Engine.ReportDocument.Export()
       at TARN.Report.ExportCrystal() in C:\Inetpub\wwwroot\TARN2008\TARN\Classes\Report.cs:line 917
  InnerException: System.Runtime.InteropServices.COMException
       Message="Missing parameter values."
       Source="Analysis Server"
       ErrorCode=-2147217394
       StackTrace:
            at CrystalDecisions.ReportAppServer.Controllers.ReportSourceClass.Export(ExportOptions pExportOptions, RequestContext pRequestContext)
            at CrystalDecisions.ReportSource.EromReportSourceBase.ExportToStream(ExportRequestContext reqContext)

This needs to go live 1st April and we didn't expect to have problems upgrading Crystal versions so it is pretty urgent now, any help gratefully acccepted.