Have absolutly the same problem.
I was running 0.8.2 and after my vServer (Ubuntu 12.04) was down (hostsystem down) i had this effect.
I decided to rollback my vServer and everything was okay again for round about 18 hours. Then i got the same effect again without any downtime of the server.
I really though it has to do something with my system i don't understand.
So i just upgraded to 0.9.0, but after maybe 32 hours i now have the same problem again.
I Figured out that the \var\lib\redis\redis.rdb is messed up.
I made a rollback just couple hours ago, but somehow it seems my rdb is not in my server backup.
When i'm back home i can load a manually created backup file of my db and that should help.
Just a question of time when it will be messed up again.
I think i will have to completly cleanup the system from nodebb and all it's required packages and make a fresh install of 0.9.0
Since i'm not at home right now i can't provide any more informations.
just saw this https://community.nodebb.org/topic/6966/lost-all-my-data-help-please/2
two topics same problem