The first day of the week is controlled by the language specified of the user login.

Default Language - "English" returns a 7 while "British English" will return 1.

You need to edit the RSReportServer.config file in the "C:\Program Files\Microsoft SQL Server\MSSQL\Reporting Services\ReportServer" folder and remove the Excel delivery extension but it does mean that nobody will be able...

I believe it's related to how/when the dataset set is used in the expression 

I take it that this is a matrix report, you could have a look at "InScope"

=iif(InScope("matrix2_GRP1"),...

Hi Eva,

Have you tried deleting the report and then re-deploying it.

Robbie