I'm having the exact same problem. Though I think I've narrowed it down to a few things. It doesn't always happen, but in Eureka (any zone) after a FATE NM dies, and everyone in the area starts to mount and leave, my game slows down CONSIDERABLY and then I get the "unexpected error" and it force closes. If I'm just doing other stuff in Eureka or there's a FATE w/o a hundred people, everything is fine. I've even tried turning my graphics down to the absolute lowest setting and it still happens. I don't think this is my rig slowing down and losing ram or something to make it quit. I know my rig can handle it, I built it in August. I've also tried a fresh re-install, and pretty much everything else OP said. For me it only happens in Eureka though. I can do literally anything else for a full day and get no errors.