A few customers are relating Google Apps authentication stopped working. The error message is the following:

Unexpected error in the authentication system

Workaround
Use the local users created by Gapps

1. Have an administrator log in using the local user
   • Use the Google username
   • Reset the password using the 'Unable to access your account?' option in the login page
   • Use the local signin, not Google login
     
2. Once the administrator has access, (s)he can reset all users password, so they can log in the same way

   https://instance.atlassian.net/admin/passwordpolicy
   

   • An alternative is to have other users reset their password as the admin did