This is the second time(and second computer) where I've had this problem in Yosemite. Basically, when I enter my iCloud username/password I'll get hung at the sign-in screen and it never will connect. I have three computers running Mavericks now associated with this iCloud account that work fine with Messages-it seems to only be Yosemite where I'm having trouble, and only recently at that. Below is the screen where I get stuck. Any thoughts on this?