System.Web.UI.UpdatePanelTriggerCollection must have items of type 'System.Web.UI.UpdatePanelTrigger'. 'asp:AsyncPostBackTrigger' is of type 'System.Web.UI.AsyncPostBackTrigger'.
I scratched my head for a little thinking this was something more complicated that it was.
After upgrading a ASP.NET site from ASP.NET 2.0 to the 3.5 framework where the site used a derived AJAX update panel in a separate referenced class library.
<asp:AsyncPostBackTrigger ControlID="albViewMoreCategories" EventName="Click" />
- was the line causing the compilation error.
The website project updated to 3.5 fine and changed the reference to the system.web.extensions class to a 3.5 version. However the associated class library still clung onto the previous 1.0 version of the class.
Removing the system.web.extensions 1.0 and relacing with it with the 3.5 version got rid of the conflict between the two projects.
I hope this helps some one.