The Notification Server and/or the Thumbnail Server doesn't start after the Upgrade to 6.12
Error message: "There is more than one database connection enabled for Notification Server. This is not supported."
"Email Notifications" / "Thumbnails" is activated for more than one database connection.
Ensure that "Email Notifications" is only activated in the notification connection and "Thumbnails" is only activated in the thumbnail connection.
You can see a faulty thumbnail connection in the screenshot below. You would have to uncheck "Email Notifications" in this case.