In order to isolate the root cause of the problem you are facing you should proceed with some testing steps:
1.Please disable all compression modules in your application and verify if the error still persists.
2.Disable the ajax by setting EnableAjax="false" for the RadAjaxManager control and verify if this makes any difference.
After sharing the results of these two tests we will be able to easily pinpoint the exact source of the issue.
the Telerik team
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