Look like mplayer is not properly installed on your server, verify with your server admin following command can convert video into FLV format.

Code:
/usr/bin/mencoder '/home/heyaheyc/public_html/video/test.mpg' -o /home/heyaheyc/public_html/flvideo/4ca4238a0b/12484961921289958177.flv -of lavf -oac mp3lame -lameopts abr:br=56 -ovc lavc -lavcopts vcodec=flv:vbitrate=500:mbd=2:mv0:trell:v4mv:cbp:l ast_pred=3 -srate 22050 -ofps 24 -vf harddup
If not, your mencoder or other related software installation is faulty.