из-за чего возникла эта ошибка? вроде бы и сбоя не было, и чистки антивирусом.

Database error in vBulletin 3.8.4: 

Invalid SQL: 

SELECT 
IF(votenum >= 1, votenum, 0) AS votenum, 
IF(votenum >= 1 AND votenum > 0, votetotal / votenum, 0) AS voteavg, 
post.pagetext AS preview, 
thread.threadid, thread.title AS threadtitle, thread.forumid, pollid, open, postusername, postuserid, thread.iconid AS threadiconid, 
thread.dateline, notes, thread.visible, sticky, votetotal, thread.attach, thread.lastpost, thread.lastposter, thread.lastpostid, thread.replycount, IF(thread.views<=thread.replycount, thread.replycount+1, thread.views) AS views, 
thread.prefixid, thread.taglist, hiddencount, deletedcount 
, NOT ISNULL(subscribethread.subscribethreadid) AS issubscribed 
, deletionlog.userid AS del_userid, deletionlog.username AS del_username, deletionlog.reason AS del_reason 

, threadredirect.expires 

FROM vb_thread AS thread 
LEFT JOIN vb_deletionlog AS deletionlog ON(thread.threadid = deletionlog.primaryid AND deletionlog.type = 'thread') 
LEFT JOIN vb_subscribethread AS subscribethread ON(subscribethread.threadid = thread.threadid AND subscribethread.userid = 1 AND canview = 1) 

LEFT JOIN vb_post AS post ON(post.postid = thread.firstpostid) 

LEFT JOIN vb_threadredirect AS threadredirect ON(thread.open = 10 AND thread.threadid = threadredirect.threadid) 

WHERE thread.threadid IN (0,9839,6392,6391,1981,1473,88,1375) 
ORDER BY sticky DESC, lastpost DESC; 

SQL Error : Table './klon/vb_post' is marked as crashed and should be repaired 
Error Number : 145 
IP Address : 90.188.220.91 
Username : xD 
Classname : vB_Database 
SQL Version : 5.1.54

Ну тебе ж пишут: Table './klon/vb_post' is marked as crashed and should be repaired.
Восстанавливай.

К слову. Тема создана в разделе по Microsoft SQL Server. Ты убеждён, что у тебя именно он, а не MySQL?