I noticed a week ago that I had an API error 503 in Mailchimp for Woocommerce. The first time I noticed it was after the following steps:
I found a fake user created on my website, which I deleted an hour after it was created
I had added Stop Scammers, then noticed that my WP core files had been modified. I am thinking possibly Stop Scammers may be written rules in my core files.
Mailchimp support suggested this might be related to Akamai when there is an issue with an IP address.
Godaddy malware and clean up scans show that site is clean, as do Sucuri sitechecker and Google Search Console. I have tried restoring the website to back ups, and uninstalling Mailchimp for Woocommerce, and also deactivating Stop Spammers.
At the moment I am reluctant to deactivate my plugins, because over the past week I have been hit with a lot of failed login attempts, and a few suspicious attempts to gain access to my site. Since upgrading my security, this has reduced to a managable 1 or 2 hits a day.
Godaddy team has advised that nothing is blocking my admin-ajax.php account.
Mailchimp has also asked me to get someone to SSH into Wordpress server and run a cURL scan on the Mailchimp plugin to see if it returns any errors.
Also I have not been able to upgrade to Wordpress 5.2.3.