i've been downloading vids from vimeo & youtube, converting them, adding a watermark, and rehosting. (user submitted videos, then i add them to a youtube channel. just clarifying so it doesn't sound like i'm ripping people's work off.

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but the last few haven't worked.
i get the error msg "can't prepare video" from mpeg streamclip.
anyone know what this is about? google says it may be a codec issue, but the thing is i can PLAY any of the downloaded .flv videos, i just can't convert them to mpeg4 & add my watermark. if there's a fix for this i'll keep using mpeg streamclip as then the watermarks are consistent. if not, what else can i use to do the same job?