SQL-DATABASE ERROR

Database error in WoltLab Burning Board: Invalid SQL: SELECT t.*,p.userid,p.posttime,p.posttopic,p.message,p.postid,p.allowsmilies,b.allowsmilies AS allowsmilies_b,b.allowhtml,b.allowbbcode,b.allowimages,u.homepage,u.username FROM bb1_threads AS t LEFT JOIN bb1_posts AS p ON (t.threadid=p.threadid AND t.starttime=p.posttime) LEFT JOIN bb1_boards AS b ON (t.boardid=b.boardid) LEFT JOIN bb1_users AS u ON (p.userid=u.userid) WHERE t.boardid IN (5) AND t.visible=1 GROUP BY t.threadid ORDER BY t.threadid DESC LIMIT 2
mysql error: Expression #22 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'grafikkreators.p.userid' 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
Date: 22.11.2017 @ 10:30
Script: /portal.php?sid=e59e366826c66a448fbc5487d13e29a4
Referer: