Google says the global authentication system outage on Monday was caused by a bug in the automated quota management system impacting the Google User ID Service. Users weren’t able to access Gmail, YouTube, Google Calendar, Google Drive, Google Maps, and several other Google services for almost an hour on Monday, December 14th. The root cause behind the outage was decreased capacity for Google’s central identity management system due to a bug. Google said that this major outage also affected the company’s internal users and tools, causing delays during the outage investigation.
Source: https://www.bleepingcomputer.com/news/google/google-explains-the-cause-of-the-recent-youtube-gmail-outage/