First, as Nitin suggested, ensure all FF instances are closed, even on the Processes tab in Task Manager.
Since the Telerik extension is correctly enabled, there may be a conflicting Add-on. Try disabling all non-Telerik Add-ons and try again. If it succeeds, re-enable those Add-ons one at a time until you identify the culprit. If you identify a specific Add-on as problematic, please let us know what it is so we can add it to our documentation.
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