[Forminator] MailChimp API key error

I’m unable to save the MailChimp API key in the Forminator Integrations. Every time I try, it returns this error:

Failed processing the request: Syntax error

I’ve ensured that there are no blank spaces or typos in the key entered, and have tested with all other plugins deactivated.

During a live chat, we also tested with the API key from Patrick that works on his sites, but it does not work on my site. I’ve also checked and there is no firewall active on the server.