It seems you are using old version. If you download latest version from mplayer site, you need the new command options.

Try change uncommenting the new and see if that works.

If not send a mail to your host with convert command and say that is not working, they will know which command work with the mencoder installed on the server.

rpm.livna.org

It can be old version, just try command for old version and see if that works.