After reading through the thread a little more carefully (actually, the first time I just read the last post ), I realised that it is indeed not a codec issue.
The problem is that a small corruption occured somewhere (probably during the download process). The corrupted data prompts Quicktime to read it as a "stop-point", hence the premature stop. Fortunately, VLC does play it, and if it doesn't, mplayer will.

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