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When using zencoder the cron job always reports back
"Could not convert video"
this is simply because it is looking for a response string like
'X videos converted successfully' but actually gets
'
X videos converted successfully'
with an extra blank line in front because in the zencoder part of method it returns an empty string from
$this->errorMsg[] = $table->getError(); after succesfully updating video status.
this is pretty easy to fix and i reported this many versions ago.