I see this occur quite often and even I keep on repeating this mistake – when it comes to setting up the Report Server URL and Report URL properties for a Reporting Services based report (in SharePoint integrated mode) in Performance Point Services Dashboard Designer.
I will quite often just copy and paste the URLs for the report when viewed through SharePoint itself, as is the case below:
As you can see this doesn't work and the preview shows the “The website declined to show this webpage” message.
The correct way to configure the Report Server URL and Report URL when accessing a Reporting Services report that is in SharePoint integrated mode is:
Report server URL: http://denalictp3/_vti_bin/ReportServer (that’s _vti_bin with underscores)
Report URL: http://denalictp3/sites/BI/Shared%20Documents/Report1.rdl.