As we are using windows identity foundation as authentication mechanism it’s create authentication cookie in HTTP header, we observe that when we run Adhoc report which is created using “GrapeCity.ActiveAnalysis.Web” it’s write data into HTTP header (e.g. Snap-Date data), result of that is HTTP header size increases and we got error as show in screen-shot (i.e. HTTP header is too long).
Please find attached database and fiddler error detail screen-shot
Hope this will help you to reproduce the issue, Please let us know if you need more details.