User command that works:
ffmpeg -f video4linux2 -i /dev/video0 -c:v libx264 -preset veryfast -f mp4 /dev/null

ffserver configuration stream that fails
<Stream test1.mp4>
# coming from live feed 'feed1'
Feed feed1.ffm

Format mp4
VideoCodec libx264

# Bitrate for the video stream
VideoBitRate 1024

# Number of frames per second
VideoFrameRate 25

# Size of the video frame:
VideoSize 1280x800

AVOptionVideo preset veryfast


# Suppress audio
NoAudio
</Stream>