I've seen the old threads on this but I don't know if there was ever a fix. I just got my Vivofit and am trying to get it all set up. I get it synced, and then it asks me to log in. That screen/pop-up is blank (white) and I can't input anything. I have the latest version (3.2.21.0) of Garmin Express. I tried doing the high contrast thing as well as uninstalling/reinstalling, but neither worked.
The blank screen is commonly caused by security hardware/software blocking the connection or by out-dated graphic drivers. Refer to FAQ: Why can't I enter my Garmin Connect credentials through Garmin Express?