This error message is quite descriptive and it is usually related to unexpected server responses to a partial postback. If a server error is thrown during an AJAX request the error message the server returns cannot be parsed and this is the message you get. Other possible reasons are listed in the description - Response.Write() for one. What I can advise is that you debug your code and see what is the exact problem and error message. If it stems from our controls and not from your custom code I advise that you first take a look at the release notes
for any related changes that may affect you and if this does not help - to send us a simple page where the problem can be observed so we can actually see and debug it.
All the best,
the Telerik team
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