The usual solution for the authentication issue is the following steps.
- Login to Netlify using the email address and password only. If you do not have a password configured, you can add one by using the password reset form: Netlify App
- Once logged into Netlify with the email address and password, change the GitHub account associated with this email address here: Netlify App
- Log out of Netlify.
Once this is done, you should be able to log into Netlify using GitHub or the email address. Both methods of login should work now.