Попробуй измени строку
exec_pull ffmpeg -i "rtsp://@10.10.120.200/0" -vcodec copy -f flv -an rtmp://localhost:1935/show/live 2>>/var/log/nginx/ffmpeg_show.log;
на
exec_static ffmpeg -i "rtsp://@10.10.120.200/0" -vcodec copy -f flv -an rtmp://localhost:1935/show/live;
