The issue stems from the fact that the attachEvent method is no longer supported by the latest IE as announced in msdn.microsoft.com
Please note that the mentioned version of the Telerik controls is no longer supported. Therefore it is expected that you might be experiencing issues related to
new browser versions. The issue you have faced should not be present in the Telerik UI for ASP.NET AJAX latest version. For this reason we would suggest that you use the latest version of the Telerik controls that are designed to be compatible with the latest browsers
For more information you can refer to Telerik Support Lifecycle Policy.
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