Ok, I finally got this GPS problem solved. After four weeks of waiting, the service called me that my N9 cannot be fixed and I got a new device as a replacement. No GPS problems so far with it! It's now working as one would expect and no maddness in syslog either anymore. As already said before, my problem was due to a faulty HW unit and was most likely else than most of the users have been reporting in this thread. No SUPL server tricks helped or anything.