Hi,
The SQL query used in JoomSEF shouldn't cause any problems. It works correctly with clean Joomla and with both JoomFish and FaLang.
I'm sorry, but I don't know what the error message "5 - cannot calculate position of lft within BY `lft` DESC" means - I think the author of the query parser should check what exactly it means and fix it.
This should be the whole query:
SELECT `id`, `title`, `alias`, `description`, language, `metakey`, `metadesc`, `metadata`, `parent_id` FROM `#__categories` WHERE `lft` <= '{$idx->lft}' AND `rgt` >= '{$idx->rgt}' AND id!=1 ORDER BY `lft` DESC
Where $idx->lft and $idx->rgt are values obtained by:
SELECT `lft`, `rgt` FROM `#__categories` WHERE `id` = '{$id}'
Where $id is the ID of the content category.
I don't see any problem with it.