Hi Retrotel,
This error message is probably referring to the filter in the Generate Report action (what records to export), rather than the dropdown filter which determines which reports are available. If you take a look at the Generate Report button on the screen, and edit the Generate Report action (bottom of the list), "resultReportFilter" is used to filter the data.
Since there are over 100 actions before the report is generated, you might have some difficulty in debugging this issue. It might be easier to create a new button or screen to generate the report. The existing button also uses an advanced script to filter the report, a regular script might be sufficient for you which will be easier to work with.
If you can explain what criteria you are filtering on, and how you want the user to select the filter values, I might be able to provide you with more recommendations on this.
-Russell