I've been using VOX for Mac for 1.5 months and I faced this issue a few times already.
Randomly one of my albums just disappear for some reason and I can't find a way to play them. I'm forced to upload them again.
I just tried to sign out (preferences, account) and the albums are back again. Anyway, its seems that something is not OK.
Thanks for the notification! We will try to reproduce it in our conditions, in order to explore this strange behavior and fix it in future updates.