I'll check the reviewer registration.
I just found odd that the user is a reviewer in that conference and is not showing up in the list.
It may have to do with this bug
which creeped up in our system.
We were having problems designating reviewers.
We applied the line in the patch, and it works now.
However, our file in classes/submission/trackDirector/TrackDirectorSubmissionDAO.inc.php is completely different, although our version is a stock 18.104.22.168
Has this anything to do with the error?