Having the same issue. Mailchimp stated:
Thank you for providing those further details and that new reference number.
We took a look more specifically into that reference number and we’re seeing that what is causing that block is that there are some SQL injection attempts being made on that IP address.
Here is an article online that talks about this further:
https://www.w3schools.com/sql/sql_injection.asp
One thing I wanted to let you know about this off the bat, the Mailchimp for WordPress plugin is created by a company called “ibericode” which is on that page you sent through, unfortunately since Mailchimp didn’t create that, our knowledge is a bit limited around that integration. It may be that the integration is what is making those SQL injection attempts or it could be another integration that is being used as well.
Unfortunately this is where our knowledge is limited since this is occurring outside of Mailchimp but what we recommend is that the hosting provider for the IP address may be able to monitor the IP activity to narrow down where that’s coming from and potentially looking into those integrations to see what may be doing those SQL injection attempts.
I hope that helps to move forward with some actionable steps, certainly if you have any questions or would like to discuss anything further, please feel free to reach out in this email and I’m happy to discuss.