We managed to observe the issue and it seems there is a bug in the control that allows drop of items that the RadDocking control cannot use. We logged the issue in our Public Issue Tracker System
where you can track its status.
I updated your Telerik points for bringing this to our attention. Unfortunately we are not aware of any workarounds to this as canceling the drop in the control concerns many built-in functionalities. We apologize for any inconvenience that this may be causing.
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