At first I tried a USB drive, but went to an SD card plugged into a reader that plugged into the USB port and voila, no more problems.
It is weird that it used to work fine but now is not. Have you recently updated your Boombox firmware?
I know someone will come along with a fix, but I've not had the problem with my '15 like you are having...yet.
All the songs on the USB are MP3 files, correct? And your file structure is all in the root directory of the memory stick?
On my SD card, every musical group is kept in their own folder and every folder is part of the root directory.
Did you use the same software to create the memory stick folders?
You get the gist...keep your methods the same when creating the folders on the memory device.
You could try pulling your main fuse and then trying to recreate the weirdness again. Also, I believe there is a reset within the menu structure of the boom box. You could look for that and try it as well.
Sorry for the random thoughts, but I'm just thinking out loud here...
