Do you see an error on top of the screen when you hit the save button?
If it’s a 503 error it means your hosting server IP is blocked by the MailChimp firewall.
Akamai is the firewall MailChimp and many other major websites use to protect their servers, which is why this is somewhat harder to fix.
When this happens, we recommend you to change your IP address because having a bad reputation at Akamai does not just affect the MailChimp API.
There are various ways to go about this.
Ask your webhost for a new IP address.
Migrate to a new webhost if your hosting company is unwilling to fulfill your request.
Configure your server to connect to the MailChimp API server using a proxy.
Hope that helps. If you have any questions, please let us know!