Using RPCS3 - Game not starting

I'm trying out RPCS3 for the first time, and to started I've downloaded Assassin's Creed from Vimm's Lair. But the game doesn't start, there's something it can't load. This is what it says:

E SYS: 'sys_mutex_lock' failed with 0x80010005 : CELL_ESRCH [1]

I'm really not computer savvy, so maybe it's a simple fix. I have installed that PS3 update that you need, but maybe there's something else I'm forgetting.