Plugin: MailChimp for WordPress, subscriptions stop working

Hi,

The site and account are not mine, this is a job for a client.
Everything was working fine until today, from one moment to another when someone tries to subscribe we get the “Oops. Something went wrong. Please try again later.”, i checked if they changed the list or the api key but everything is in order.

I have already disabled all the plugins, cleared the cache and all the normal solutions.

I ran a test with a new list and it worked for a moment, after 4 or 5 subscriptions start to show the error again. The list did not got to the maximum subscriptions, in the mailchimp admin i can add and delete subscriptions.

I have another site from another client in which the mailchimp account and plugin are working fine.

I dont know if is relevant, they imported some subscribers on the list (i don’t know anymore than that because i didn’t do it).

This is the site in which i have the problem
http://www.upstartco-lab.org/
“Sign up for our blog!” in the footer.

Sorry for my english.