Which version of RadEditor for SharePoint do you use in your SharePoint site and under which scenario? My suggestion is to upgrade to the latest one 6.3.1 and try to reproduce the problem again. We will also release next week version 6.4 and you can test it once it becomes official.
If you are using the standard RadEditor for ASP.NET AJAX in SharePoint 2010, than our recommendation is to migrate to the especially designed for SharePoint usage RadEditor for SharePoint 2010.
If the problem still persists, zip the Program Files\Common Files\Microsoft Shared\Web Server Extensions\wpresources\RadEditorSharePoint folder and send it for examination along with the web.config file of your SharePoint site. You can use the support ticketing system to attach zipped files.
the Telerik team
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