Very fortunately I started on this trace you suggested. I tried to merge the nathelper.cfg file (shipped with openser as a factory documentation) with my openser.cfg. And voilá, it seems to work smoothly! :-D

I report that everything works now well. Only one thing remained: the media stream is not put to the directory I entered. What's more, I cannot see any real open files in /proc/`cat /var/run/rtpproxy.pid`/fd --- just a few links to sockets. Am I missing something here?