Page cannot be displayed message

Hi,

When I run a webi report in the interactive viewer in XI 3.1 I get a 'The page cannot be displayed'. When I look at the technical information on that error page it says " Error code: 500 Internal Server Error. The request was rejected by the HTTP filter. Contact the server administrator (12217) "

Sometimes a report will run and be displayed OK but I seem to be getting this message on longer running reports.

Does anyone have any ideas why this is?

Thanks,

Chrs