Sunrise, we don't really support H.264 in AVI. Putting H.264 in AVI is a bad idea. The presentation timestamps are missing in such case and our pipeline doesn't currently restore PTS for H.264 (since in *all* other containers the timestamps are present). I'd suggest to remux the file to MKV using mkvtoolnix, after which the file should play.
Yes, you're right, that didn't really cross my mind, I was just a bit confused that VLC and LAV played it fine, but I didn't really realize the fact that it's actually H.264 in AVI and not MP4 (you can see in my above post that I even confusedly wrote .MP4).
H.264 in AVI is a bad idea, I fully agree, not sure why such files even exist in the first place.
BTW, I have just seen in the ffmpeg changelogs that ffmpeg now directly supports NVENC from Nvidia. Does this mean (were you using ffmpeg directly in combination with gstreamer or was is libav?) that we also get Nvidia Hardware Encoder support sometime in the future?