Are you sure ConvertTube_com.mpg is not corrupt (i.e. can you play it on your computer)?
Otherwise, there may be something wrong with the way ffmpeg is set up on your computer, and I'm not even going to start to troubleshoot that since I've never even used ffmpeg. However, the following mencoder (comes with
mplayer) command has handled every file I've thrown at it:
mencoder -of mpeg -oac lavc -lavcopts acodec=mp3:abitrate=192 -af resample=44100:0:0 -ovc lavc -lavcopts vcodec=mpeg2video:vbitrate=200 -vf scale,harddup -ofps 20 -zoom -xy 160 ConvertTube_com.mpg -o thevideo.mpg
Most options there are equivalent to those in the ffmpeg command, but there's a new one:
-ofps 20 means the output file will be at 20 frames per second. You can probably omit this if you want, I only use it (with a value of 12) because the iPod 5G is too slow to run at fullscreen faster than 12 FPS.
and a slightly different one:
-zoom -xy 160 means the video will be scaled to 160 pixels wide (height is automatically calculated).