asp:XXXXX is not a known element. This can occur if there is a compilation error in the web site.
When working with the RC version of the ASP.NET AJAX Library, I was getting this error with the updatepanel controls and the scriptmanager control. I followed the Microsoft documentation and renamed everything to the asp prefix, that belonged to the ajax library. However, I kept getting these errors after I upgraded from beta 2 to RC. So what was the problem?
After digging around for a while, I found out on an ASP.NET forums post that the asp prefix is the problem. After changing it to ajax, it works fine. Note that you can have multiple controls use the same prefix (as long as there isn't a name resolution problem), but you can't use the 'asp' prefix. I understand that the microsoft team is getting users ready for when the AJAX library is part of the .NET framework (in the future); however, that was a horrible problem, as it affected how my code got rendered and messed with the designer.