I had back this problem a 2nd time 2 days ago, I don't know if it's related but I was doing iptables FW settings on the VM, I had nodeBB running, and also redis and from my computer desktop redis desktop manager connected to DB.
Then I made a mistake in iptables, blocked all redis port (even local), saying nodebb unable to connect to redis (so as redis desktop manager) then when I openned back 6379 port, all data was gone, not sure if it's related, just an information.