I can't tell you how long we've racked our brains trying to figure this issue out. We are using the Bootstrap theme for RadListView and are doing drag and drop. We've been getting a "This is an invalid webresource request. " error when first dragging an item on the page.
I just came across this article which provided me some new guidance:
And we decrypted the querystring of the WebResource request and it is looking for this:
When we change our RadListView skin to "Default" or "Black", this drag and drop error is gone so there clearly must be a broken link/reference for that image because we also see the little drag and drop cross overlaying the items we are dragging.