Thank you for the question.
When you are dragging from one listbox to another you are attempting to remove an item, which however is bound to a data record and if you remove it you will leave RadListBox in an invalid state, that is, you will desynchronize it with the data source. There is no way to fix this behavior of RadListBox.
What you need to do is create a new RadListBoxItem with the same information as the item that you are trying to drag and drop in the destination list box. You can either remove the data record for the original item from the data source, which will also remove it from the source list box, or you can simply hide it and pretend that it is deleted. You can hide it by setting its Visibility property to Hidden.
Write again if you have other questions.
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