Users with Google Apps integration cannot log in to their instance

Update

This issue has been recognised by Google Engineers as an issue with Google Authentication and Google are actively investigating the issue.

The workaround being suggested by Google is still the same: "Re-authenticate the application", which means disconnect from Google Apps integration and re-connect.

*http://stackoverflow.com/questions/40488769/google-oauth2-tokens-invalidated-more-and-more/40521704#40521704*

Summary

When having Google Apps integration enabled in your Cloud instance some users cannot log in and they get error: 'Unexpected error in the authentication system'

Environment

Atlassian Cloud with Google Apps enabled

Steps to Reproduce

1. Access your cloud instance
2. Try to log in using your google account

Expected Results

Log in successful

Actual Results

Error 'Unexpected error in the authentication system' is thrown. No access is possible. This only affects a limited number of customers.

Workaround

Ask one of your site-admins in the Cloud instance and admin in the Google domain to disable/enable the Google Apps integration in the user management. Further details on the how to do this can be found here: Integrate with Google Apps

To login to your Cloud instance when Google Apps isn't working you can use the standard login using your email address and password prior to Google Apps being synched. If you're unsure of your login credentials use https://<instance>.atlassian.net/login/forgot to recover them.