Anyone who currently gets a error message when they log in to Google Calendar is not alone. The service has apparently failed worldwide. This is indicated by reports in USA and Europe . Also Japan India and South America are affected. Also in Austria and Germany many do not have access to their calendar data. You get a 404 error and a page not found, as well as the futurezone could check in a short test. Sometimes 503 is displayed as an error.
What could be the cause of the major failure is unknown. Recently, there were reports that fraudsters with a spam phishing wave led Google Calendar users astray. Whether the current failure has anything to do with this is speculation at the present time. Also on Twitter the failure is commented on practically every second in various languages and with various GIFs. The official Twitter account of the Google Calendar has since confirmed the problem. Work on a solution.