Run the command

Code:
/usr/bin/mencoder '/home/amatecom/public_html/video/MY-WIFE-AND-MY-FRIEND-1.3gp' -o /home/amatecom/public_html/flvideo/1240602928501300864.flv -of lavf -oac mp3lame -lameopts abr:br=56 -ovc lavc -lavcopts vcodec=flv:vbitrate=500:mbd=2:mv0:trell:v4mv:cbp:last_pred=3 -srate 22050 -ofps 24 -vf harddup
in SSH and see if it is able to do the conversion. If you don't have ssh, contact your web host/server admin.