Howdy hmc,
I am sorry about the delayed response. I typed one up a few hours after you posted, but I must have exited the window too quickly after hitting submit because it’s clearly not here now. 🙁
I have not seen that problem before. And searching through the site I can not find a single other user of that plugin, at least one who has mentioned it by name.
If that plugin restricts the wp-admin/ directory, that is probably the cause. wp-admin hosts one file that non admins access all the time, admin-ajax.php. For whatever reason WP has put the ajax API inside of that file. So if you restrict the directory, it breaks ajax WP wide for anyone that hits the restriction. If this is an option inside of Restrict Content, disable it so that everyone can access wp-admin.
Did that help at all? Please let me know. Cheers!
– Brook