Mailchimp Sync API Key test failing


I'm continuing to set up my new membership site using Multisite/Buddypress and need to integrate my Mailchimp list. Having installed the plugin, I enter my API Key but it fails. I even tried creating a new key but to no avail.

In the log it says:

[CODE:http_request_failed] [EMAIL:] - SSL certificate problem: unable to get local issuer certificate

I thought it might be because Mailchimp might insist on https, so I installed an ssl certificate but still nothing. Incidentally, how do I make WordPress use https on login?

Thanks in advance and Happy New Year!