Thank you for your quick reply.
I understand that it’s a standard HTTP status. When I disable the plugin and the students go to the home page and then log in, the error does not appear. Maybe because they are all logging in at different times and not all at once, but the error seems to disappear.
I created 40 separate, unique accounts for each student.
It is a managed WP site through GoDaddy.
I’ve moved the site to a private server and was hoping to use the plugin on that site, but wondered if this was happening to anyone else.
From your reaction, it would seem I am the only one who has had this problem.