Fixed
My friend solved it by stripping invalid utf characters in the body of the message.
Here's the code.
in file '\include\class.ticket.php' around line 628 :
$sql='INSERT INTO '.TICKET_MESSAGE_TABLE.' SET created=NOW() '.
',ticket_id='.db_input($this->getId()).
',messageId='.db_input($msgid).
',message='.db_input(Format:(iconv("UTF-8","UTF-8//IGNORE",preg_replace('/','', $msg)))). //Tags/code stripped...meaning client can not send in code..etc
',headers='.db_input($headers). //Raw header.
',source='.db_input($source).
',ip_address='.db_input($_SERVER);