Basic Wordpress blog subscription form not working

I have the basic blog subscription plugin activated on subsites. On one particulary subsite, I am getting this error message: "There was an error when subscribing. Please try again."

Other subsites' forms are working. Can you try to subscribe at this subsite: Here is the URL after attempting to subscribe: (in case this gives you any clues).

I am granting access if you'd like to take a closer look.

Thanks, Joe.