The server mostly just acts to keep players coordinated - it doesn't act as a centralized game manager like in, say, most MMOs. The gameplay control (object management, creature simulation, etc) for any given empty zone (64mx64m) is actually handled by the machine of the first player to come close enough to render it, so if one of your players has a particularly weak machine or poor internet speed/stability and they happen to be that "first person to enter the zone", then this is what you're going to see.