Okay so at first I was annoyed that mine wasn’t working, but I found a way. You need to copy the map and the build data, and then rather than double-clicking you need to open them manually e.g. with “File > Open Level”. That’s what mostly works for me anyway.
I very rarely comment but losing my work a few days from deadline makes me feel a little hurt, and now having a backup open makes me feel very relieved.
I can’t guarantee it’ll work, for anyone coming across this thread after Googling like I did, since a few of my map backups appear unusable.