When I try to log in with an email address that has "+" in the name, it then asks me to sign up, then when I try and sign up, it says I already have an account and says to login. Thus creating a loop.
Have they pushed an update disallowing "+" in email address? Last Play Store release was in April? So that last major update which changed the way you login (from password, to email verification), might have set them up to shortly thereafter disable "+" at serverside?
Out of curiosity, why did you have a ‘+’ in your email address?