I too had the security error a few times earlier today but eventually managed to get some withdrawn. Just stick at it if you REALLY need to withdraw.
Those guys have their work cut out with demand growth over the past week or so. Sure they'll sort it, just hope I can get coins in and out when I need to without too much of a wait. I've given up trading inside it for the moment, taking too long to be usable. Can't remember the last time I got the balance page to load successfully
That said, my general experience has been positive since joining it.