I watched this serial as a kid sometime in 80's in India. It is an excellent TV serial. The original serial is in German, I believe, but it was telecast translated in English. The dubbing was so good that I did not even realize that it was not in English originally. (Of course, I was a kid then). In particular, I remember an episode where the killer used to kill his victims by giving poison in the chocolate. Very nice performance by Lowitz.
I have been looking for its DVD (in English, of course) but have not been successful in finding it. If anyone has it or knows where I can get it, please let me know. If you ever get a chance to watch this serial, please do not miss it. I am sure, you will like it especially if you are fond of detective serials.