Never had this issue before. Haven't logged in to make any changes in about 9 months. I've always used the Google sign in process without issue. My password and user name are definitely correct. When I hit sync on the I Phone I'm directed to a page that tells me to be sure cookies are on. They are on. I have a Google+ profile but unsure if that matters based on forum entry's. Forum entry's on this subject don't help much.
I'm using app version 4.7.0 (4771) on an I Phone 6.
Please tell me how to log into your system.