I got a response for Garmin, that the "use map" setting is the official solution to the problem... I don't get it, it worked so well before and now it was broken....
actually, there is a workaround, as the problem obviously lies somewhere in garmin connect and importing the course to the watch. Using hints from another thread I manage to create a course with turn notifications and upload it using cable as fit file to the watch. And it works as it worked before.