On Monday afternoon in the United States internet administrators started reporting on the internet outages list that users were unable to log in to Google services. While this wasn't happening everywhere nor to everyone -- I was able to log in from San Diego, CA -- the problems appear to be wide-spread.
Google has confirmed that many users are unable to log in. According to the Google Cloud Status Dashboard, "We are currently experiencing an issue with authentication to Google App Engine sites, the Google Cloud Console, Identity Aware Proxy, and Google OAuth 2.0 endpoints."
OAuth is the foundation for Google Sign-In services. This is used when you sign in to such Google services like Gmail, Google Calendar, and Chromebooks. OAuth is also used by other applications when you use Google to log into them. When OAuth is knocked out, you may be unable to log into any of these services.
Google is reporting progress in fixing the problem.
"Mitigation work is currently underway by our Engineering Team to address the issue with authentication to Google App Engine sites, Google Cloud Console, Identity Aware Proxy, and Google OAuth 2.0 endpoints. Error rates are dropping and we're seeing service improvement. We will provide another status update by Monday, 2019-08-19 13:45 US/Pacific with current details."
In the meantime, some users have reported they've been able to log in by using an incognito window under the Chrome browser.