Replace that line with the following:

$DB_site->query("UPDATE forum SET lastactivethread = LEFT('".addslashes(htmlspecialchars($threadinfo[title]))."',$lastactivethread_length) WHERE forumid = ".$threadinfo[forumid]);

It's the extra dot in front of the addslashes that's causing the problem.

Sorry about that. I've updated the docs.

It won't affect anyone who did an upgrade. The error wasn't there in the previous versions. Only those who have done a fresh install of 2.4 will see this error.