Yosemite Messages blank window
I was certain this problem was related to two factor authentication on my Apple ID because problems started happening when I tried to login. After more investigation it turns out it was a font conflict problem preventing the UI from starting.

I found an error message in the console application regarding NSString which, after a lot of googling indicated a font problem. I then cleared the font cache and reset all non standard fonts in font book and restarted OSX. The app now works