Shouldn't:
// decode pass, revert and replace key
$_POST['_pass'] = str_replace('*yourkey*','',base64_decode(strrev(get_input_value('_pass', RCUBE_INPUT_POST, true, 'ISO-8859-1'))));
be:
// decode pass, revert and replace key
$args['_pass'] = str_replace('*yourkey*','',base64_decode(strrev(get_input_value('_pass', RCUBE_INPUT_POST, true, 'ISO-8859-1'))));
??