In order to keep the module working from backend I had to extend modules/mod_jak2filter/elements/k2extrafield.php with:
case 'textarea':
case 'jak2depend':
$alias = json_decode($option->exvalue);
$alias = $alias[0]->alias;
$db = JFactory::getDbo();
$query = $db->getQuery(true)
->select('a.id, a.title, a.level, a.lft, a.rgt')
->from('#__categories AS a')
->where('a.parent_id = 1')
->where('extension = ' . $db->quote('com_jak2filter'))
->where($db->quoteName('alias').' = ' . $db->quote(strtolower($alias)));
$db->setQuery($query);
$cat = $db->loadObject();
if ($cat) {
$typeOptions[] = JHTML::_('select.option', $prefix.'textfield', JText::_('FILTER_TYPE_TEXT_FIELD'));
$typeOptions[] = JHTML::_('select.option', $prefix.'jak2depend', JText::_('FILTER_TYPE_DEPEND_RANGE'));
break;
}