This is quite unexpected.
Properties that may affect the DragDrop are IsHitTestVisible
, which should be true for the DropTargets and their visual ancestors.
Also, capturing the mouse on MouseDown or during DragDrop may affect the DragDrop.
Is it possible that the AllowDrag / AllowDrop properties were set in a style and it is not applied anymore or it is being replaced with a different style now? This will happen if you are using a custom theme and an ItemContainerStyle to set the DragDrop properties.
Can you identify what kind of changes were made? More information about that will help us narrow down the cause for the unexpected change.
the Telerik team