If the server returns an unexpected result when a Node with ServerSideCallback is expanded, the "error on callback" error is thrown. This usually happens when there is an exception thrown on the server and ASP.NET returns an error page. You could either try attaching a debugger and catching the exception to see what's causing the problem or you could try using Fiddler
to inspect the server response and see what is the error page returned by the server.
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