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: http://katharinedavis.com/. Here is the URL after attempting to subscribe: http://katharinedavis.com/?subscribe=error#blog_subscription-4 (in case this gives you any clues).
I am granting access if you'd like to take a closer look.