So you and your partner both have a Moto E 4 Plus, on Android 7.1.1, and only one of you can log in?
This being the case, I would log in on the working device, and choose the change password option, and do it very carefully, so as not to make things worse. I would then choose log out, on any and all devices still logged in, I don’t suppose there will be any, other than the working one, and then log in again with the new credentials on both. If only the same device still works, and the other not, I would consider a factory reset on the device that is misbehaving. It might be that it has a security issue. Does it give any warnings when you try to log in?