I received this error on a page which had previously been working. That morning I had updated our site, so I was expecting some issues, but this error doesn't really tell you much. We have a maintenance screen which displays a list of records in a grid. When you click on an edit link the grid converts into an "in place" editor (a UI metaphor I thought I'd like when I initially put the form together but which, as it turns out, I really hate).

A user had reported that when they clicked on a button which saves this record that it wasn't closing and updating the list. We are using Telerik web controls for most of the site so I figured it had to do with them since I updated a number of pages, including our master page, to start using the RAD AJAX version of the controls instead of their previous version. It seems this error was being caused because I had added an instance of the RadAjaxManager control to the master page, and I already had an instance of a AjaxManager (older version) on the content page itself. Both managers were conflicting with one another, which caused this error.

All I needed to change was the "RadAjaxManager" markup code to a "RadAjaxManagerProxy" and that resolved this particular error.