Hello Thomas Ossowski,
Thank you for writing.
There must be something in your code that prevents the DragOver event from firing. DragOver is an event declared on the Control class level, RadTreeView has no code which can prevent it from firing. If you are using RadTreeView's drag and drop, however, you must use only them, they do not work with the standard OLE drag & drop mechanism.
If you still believe that RadTreeView is misbehaving in some way, I would kindly ask you to post a code snipped which demonstrates what you are doing so that I may assist you further. The other available option for you is to open a new support ticket and send me a sample project.
the Telerik team
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