I'm testing this plugin as I will need it for a client website.
I've generated a couple of Api in my mailchimp account and they are not accepted by the plugin.. It keep saying: Failed - Please check your key and try again.
I have the last version of wordpress and of the plugin.
The only thing I can think about is that the test website is hosted on a free hosting provider (www.altervista.org), and I have installed my wordpress from their admin panel, so maybe it's blocking the api?
Thank you for helping!