Mysql error when opening product or category in back-end

When i try to edit a product or category, i get this error in return:

Warning: mysqli::query(): (21000/1242): Subquery returns more than 1 row in /home/megasmoke.nl/public_html/system/library/db/mysqli.php on line 18
Fatal error: Uncaught exception 'Exception' with message 'Error: Subquery returns more than 1 row<br />Error No: 1242<br />SELECT DISTINCT *, (SELECT GROUP_CONCAT(cd1.name ORDER BY level SEPARATOR '&nbsp;&nbsp;>&nbsp;&nbsp;') FROM oc_category_path cp LEFT JOIN oc_category_description cd1 ON (cp.path_id = cd1.category_id AND cp.category_id != cp.path_id) WHERE cp.category_id = c.category_id AND cd1.language_id = '1' GROUP BY cp.category_id) AS path, (SELECT DISTINCT keyword FROM oc_url_alias WHERE query = 'category_id=59') AS keyword FROM oc_category c LEFT JOIN oc_category_description cd2 ON (c.category_id = cd2.category_id) WHERE c.category_id = '59' AND cd2.language_id = '1'' in /home/megasmoke.nl/public_html/system/library/db/mysqli.php:40 Stack trace: #0 /home/megasmoke.nl/public_html/system/storage/modification/system/library/db.php(84): DB\MySQLi->query('SELECT DISTINCT...', Array) #1 /home/megasmoke.nl/public_html/admin/model/catalog/category.php(200): DB->query('SELECT DISTINCT...') #2 [internal function]: Mode in /home/megasmoke.nl/public_html/system/library/db/mysqli.php on line 40

There are certain categories and products where this error doesn't come up.

Does anyone know how to fix this problem? Thanks in advance!