Hi again! I managed to watch all kind of media files e.g. mkv, mp4 with external .srt subtitles with ffmpeg and iptvplayer installation following the instructions of this page: https://wiki.openpli.org/Plugin_Wiki:Iptvplayer. I rebooted the box opened iptvplayer, installed all recommended packages and extplayer3, opened local media from usb with MEDIA addon and now everything's o.k. No need for mediaplayer and gstreamer.