I presume your "convert to MP4" operation is just a container change, not a full re-encode of the A/V data streams. So perhaps FFmpeg is getting hung up on something broken in those AVI containers. This line in your output might support this:
[mpeg4 @ 0x17cf770]Invalid and inefficient vfw-avi packed B frames detected
You could try creating a new AVI container -- let FFmpeg copy the entire file, eg with:
ffmpeg -i input.avi -vcodec copy -acodec copy output.avi
This should create a new AVI container and copy the data streams. Then try your command again. If it works, you've got broken AVIs (or possibly an FFmpeg bug) -- either way, you can put all your videos into new AVI containers and get it done that way.
If that doesn't work, you might look further into the broken XviDs and see what they have in common.