I know there are plenty discussions about this, but I've never had this problem until recently. My MBP works fine at home, no dropouts. However, at University it drops out all the time, and half the time I can't reconnect so end up turning AirPort on and off. The University WiFi uses some sort of proxy and requires authentication. I accept the certificate, and all works fine, then a couple of minutes later it will drop out, and 1/3 times it'll reconnect. My mates laptops work fine (Windows XP, Dell's), so it is an OS X/MacBook Pro issue. However, I got this error message that I've never seen before.