Export Playlist was the right command. And it copies only the music, not the list. So it cannot be the list itself that is the problem.
I think you're going to have to look at the USB disk and see what all is there. And make sure that all the filesizes are reasonable, ie non-zero.
The fact that the B2 exhibited similar strange behavior makes me wonder if something has gone wrong with the Playlist itself. Another thing you could do is to copy the playlist (just the list, not the music) to a USB disk and look at it with a text editor. Use the Playlist to C command (USB menu). That command allows you go specify which playlist you want to copy, without getting all the others.
Please let us know what you discover.