Gerry

Okay, I managed to resolve this issue. Please make note of this for the future, since this seems to be a recurring issue (I have found several similar threads).

I had enabled password protection on the staging (test/development) version of the site. Apparently, this also blocked the MailChimp plugin’s traffic. In the future, you may wish to ask if the site in question has password protection (for viewing, not just for admin access). If so, it must be disabled for the plugin to communicate properly with the MailChimp servers.

I did a further check, after reading another person’s thread on a similar issue. I installed and activated the plugin on my live site. I was able to easily make the connection from that site – the same experience of the other user.

This implies that there is an issue with using the MailChimp plugin on sites using WordPress staging. This means I cannot implement the MailChimp link except on an actual, live site. Since I develop every page on the staging site, then make that site live, this is a significant limitation on our standard process.

Can you think of why this issue might exist only on sites not set up as “live”?

I did some additional investigation…

I talked with the folks at Siteground’s support group. They cannot whitelist incoming traffic on shared servers, but can do so for outgoing traffic if you give me the ports. They feel pretty sure it’s not a firewall issue, though I don’t know what they’ve based that on.

I disabled all of the other plugins, but am still having the same issues.

Thanks for the help, Dominic. What ports should I have them open for those addresses?

I just went through and updated the staging site to the latest version of WP (and updated all plugins). This did not fix the error.