At around 1:00 the Website was returning 500 errors.
Reason: An incorrectly configured automatic-update-system, updated the PHP version, without updating it in the webserver config. After that, it uninstalled various PHP modules from the current, older PHP version, e.g. the database connection module.
Also the Uptimerobot was set to a subdomain, which wasn't affected by the error, so it didn’t report any errors, which lead to the late detection of the incident.