Yosemite Messages blank window Messages on Yosemite used to work fine until this morning, repeatedly asking me for my Apple ID password. Months ago I had setup 2 factor authentication for my Apple ID, but it never complained. So I set up an app specific password, Facetime took it no problem, Messages didn't and kept asking for my password. I quit, log out, restart, disable/enable from my phone, nothing works. So I disable 2FA, try again and now Messages gives me a blank window when it starts.

Is there any way I can reset it somehow ?
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