One of the other development leads here found some info on the web that might be related to your problem. Personally, I haven't seen this issue before.
Is the Website running as a .Net 1.x website rather than a .Net 2.0 website? Here is a link that talks about it and a resolution:
If you open IIS, and open the properties for the "TrafficAnalysis" folder, then click on the "ASP.NET" tab, it should tell you the ASP.NET version. On my install, the combo box is grayed, so I can't change the version. But if I open up the properties for "SolarWinds NetPerfMon" website, on its "ASP.NET" tab, it does let me change the ASP.NET version. Let me know what you find out, but please also read the posts on the weblink.