We are currently improving the Drag and Drop implementation of our RadTabControl (the control which RadDocking is based on) and I hope this issue will not be present in the Q3 release. Root cause for the issue is setting the AllowDrop property of the control into the constructor. You could try to set AllowDrop to false after the control is initialized.
Hope this help. Please let us know if need further info on the topic.
the Telerik team
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