[bumblebee]

If it happens at the same song, it might actually be the song previous to this (or after). I would run a test by taking out a few songs before (after) that one and see what happens.
I presume there is enough space for it to create the backup wherever it is saving it to?
Also you could try deleting the companion app and downloading a fresh copy.
I do not know if it uses a cache while backing up and if it has left a remnant of it somewhere when it stops (Mike can tell us) but it might be causing some kind of problem if it does not start a new one each time you try the backup.