Trying to work around the crashing-bug where you move a file from one Thunar window to another and the application crashes.

So I tried doing it from thunar tab to thunar tab instead.

This time it didn't crash, instead it just wouldn't let me to the action.

The solution is to use a terminal window and copy them with commands. Incidentally, thunar doesn't refresh after an operation from the terminal. You have to manually refresh the windows.