SQL error?

[CarolSEL]

We're getting continuing error notifications when the site owner and other members attempt to log in. New registrants often cannot log in. The message is:

Database error in vBulletin 4.2.4:

Invalid SQL:
SELECT COUNT(*) AS num
FROM vb3_moderation AS moderation
INNER JOIN vb3_post AS post ON (moderation.primaryid = post.postid)
INNER JOIN vb3_thread AS thread ON (post.threadid = thread.threadid)
WHERE type = 'reply' AND forumid IN ( );
MySQL Error : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 5
Error Number : 1064

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MySQL version 5.6.35
PHP version 5.5.38
Server Type: Linux
Web Server: Apachi (cgi-fcgi)

Host tells us it's from "poor coding" in the FractalizeR notifications plugin. When we turn that off, Mods don't get notifications. There aren't any coders among us, so I have 2 questions:

1) What is the syntax error that is producing the empty parentheses?
2) In which file will we find it?

Thanks for any help!!