what's the email address you are testing with? That sounds like some kind of validation error, but I'm wondering if it is coming from Joomla or from Mailchimp.

Hi Daniel, thank you for your quick reply. The error appeared while testing the system. I was repeatedly adding and canceling myself to understand how an address was taking into account by joomla and by mailchimp. It appears that the address was already present in one of the two databases. Maybe it was a matter of cache. Clearly, the message should have been different, but the problem seems to be over.
All the best
Alberto