As you set the HasDocumentHost property of the RadDocking control to false, the DocumentHost area doesn't exist anymore. When you click the Tabbed document menu item, it sends the pane to the DocumentHost area which is not visible and that is why the pane disapperas. When you don't have DocumentHost in your docking control you should set the CanDockInDocumentHost property of all the panes to false.
Hope this helps.
the Telerik team
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