SQL-DATABASE ERROR

Database error in WoltLab Burning Board (2.3.3): Invalid SQL: SELECT COUNT(t.threadid) AS threadpostcount, t.starterid, u.userid, u.username, b.boardid FROM bb1_threads t LEFT JOIN bb1_users u ON (t.starterid=u.userid) LEFT JOIN bb1_boards b ON (t.boardid=b.boardid) WHERE countuserposts = '1' AND t.starterid != 0 GROUP BY t.starterid ORDER BY threadpostcount DESC, u.username ASC LIMIT 0,5
mysql error: Expression #5 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'ttt.b.boardid' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql error number: 1055
mysql version: 5.7.27-0ubuntu0.16.04.1
php version: 7.0.33-0ubuntu0.16.04.6
Date: 18.10.2019 @ 23:17
Script: /board/statsinfo.php
Referer: