@sergio Workaround still works (using DAVx⁵ 2.2.1-ose and OpenTasks 1.1.15), however, once I check a task as done in OpenTasks the issue returns with the next sync.
Now I’m using aCalendar+ as calendar app and OpenTasks to sync tasks. When I get a notification for a due task, I just check it as done in the aCalendar+ notification. Notifications from OpenTasks are deactivated. A sync thereafter does not trigger Vibration.
So the Vibration only happens after checking within the OpenTasks notification.