Revisions and Commits
Yeah, same here. Both videos are of similar size (1.6MB vs 2.1MB) and both appear to legitimately be MP4 / H264 videos (neither is some kind of obviously spooky magic like an AVI named .mp4).
My best guess is that this might be a general Content-Range issue (see also T8266). I know our Content-Range behavior for encrypted files is wrong in some cases (T12079) and all files on this install are encrypted, but I believe there's also some kind of bug even in the unencrypted case.
I think that the problem is the content itself. This is evident by thumbnail fails to be generated when the second one is ok. On my OS/browser combo (Gentoo/firefox) first video loads very slowly while the second is perfectly ok. On Chrome first video refuses to load.
After downloading and testing locally, everything is fine. My guess is that chrome and firefox can guess proper values for decoders for "normal" video, while for "weird" video firefox tries it's best and chrome just gives up.