Not sure why you get the error message but you can set the default search terms via the config file:
// Defaults of the search field configuration.
// The array can contain a per-folder list of header fields which should be considered when searching
// The entry with key '*' stands for all folders which do not have a specific list set.
// Please note that folder names should to be in sync with $config['default_folders']
$config['search_mods'] = null; // Example: array('*' => array('subject'=>1, 'from'=>1), 'Sent' => array('subject'=>1, 'to'=>1));
You could remove items from the dropdown via jQuery perhaps:
$('#s_mod_subject').parent().parent().remove(); // removes the subject checkbox from search options