This is a known issue with the default configuration of the RadEditor provider for DNN. You need to remove the XHTML validator module from the editor tools file, since it is not compatible with the DNN URL rewriter. Open the editor's tools file at ~/controls/RadControls/Editor/ToolsFile.xml and remove this line:
module name="RadEditorXhtmlValidator" dockingZone="Module" enabled="true" visible="true" dockable="true"
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