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<messages>
<message>Could not convert video</message> <<<======
<message>No temporary videos to delete.</message>
<message>No files to transfer.</message>
<message>No Videos to transfer.</message>
<message>No avatars or cover of users needed to be transferred</message>
<message>No avatars or cover of groups needed to be transferred</message>
<message>Removed Pending Invitation for Past Event</message>
</messages>
you can see the path of that ffmpeg, the path not same like you put at jomsocial > videos > settings please put that path from "which" to jomsocial setting.
and type this command
ffmpeg
you can see ther version. your ffmpeg version is too old: ffmpeg version 0.7.16, Copyright (c) 2000-2013
not support for video converting.