Here's what it looks like:

You won't believe how easy you can fix this :)

Just quit WALTR 2 and relaunch it. Voilà!

However, something else could be causing this problem. It might happen, when you drag-&-drop the file which is being played back in some media player at the same time (e.g., QuickTime, VLC, iTunes etc.).

That is why we strongly advise you make sure the file is not being played back at the moment of transferring.