SQL-DATABASE ERROR

Database error in WoltLab Burning Board (2.1.5): Invalid SQL: SELECT a.replycount AS replies,a.boardid AS boardparentid,a.threadid AS threadid,a.topic AS threadname FROM bb2_threads AS a,bb2_boards AS b WHERE a.boardid = b.boardid ORDER BY a.replies DESC
mysql error: Unknown column 'a.replies' in 'order clause'
mysql error number: 1054
mysql version: 10.1.44-MariaDB-0ubuntu0.18.04.1
php version: 5.3.29
Date: 25.09.2020 @ 09:30
Script: /statistik.php
Referer: