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?
TIA and best regards, Zoltan