Mailchimp Api Not Working

I've run through the KB, checked, rechecked and checked some more...and still not working.

Still shows 'Enter API Key and Save First' in the List drop down under the MailChimp tab in Email marketing addon setup even after entering and saving the API (verified) key multiple times. It's not receiving the lists I've created in MailChimp.

I've searched here and even my Googlefu is failing me. Is this addon no longer supported or working or is there something I'm missing? I've disabled the Newsletters addon as well.

Whenever I enter the API key and hit save it takes me to a blank screen.

Anyone have any input?

OK so I had to uninstall another Mailchimp add-on I had been using (I already disabled it, but actually had to uninstall it). After that, hitting "save" didn't send me to a blank screen. It brought up my Mailchimp lists into the dropdown like it should.

But now I'm getting the error message below whenever I choose a list and hit save:

ErrorWe couldn't verify the URL is working. Please double check and try again. HTTP Code: 403

It occurs to me now that it's probably because the URL it formed there is http and my site is actually all https. How can I fix that?