Still having this issue and I've tried disconnecting and reconnecting the MailChimp account, adding the site to MailChimp and still continue to have issues. The errors are random .. maybe 1 out of 4 new subscribers.
One SquareSpace tech support said "MailChimp thinks its a spam account" ... I went to MailChimp but they don't offer support to free accounts. 😒
Anybody find a workaround?