SQL-DATABASE ERROR

Database error in WoltLab Burning Board (2.3.6): Invalid SQL: SELECT COUNT(threadid) FROM bb1_threads WHERE boardid='68' AND important < 2 AND visible = 1
mysql error: Can't open file: './usr_web1_2/bb1_threads.frm' (errno: 24)
mysql error number: 1016
mysql version: 5.5.47-0+deb6u1-log
php version: 5.3.3-7+squeeze29
Date: 03.07.2020 @ 13:52
Script: /board.php?boardid=68&daysprune=1000&sortfield=lastposttime&sortorder=DESC&page=3
Referer: