Why did two of my clients get this error this week? "Mailchimp didn't save your recent Form Block submission"

Anyone know what's going on? One client recognized the person, who is a regular customer. One is from the home page or pop-up (I can't tell), and the other is from another page (definitely not the pop up or home page.) 

Mailchimp said they can't help with messages received from Squarespace. 

I'm not sure why this happens, but I have found that Mailchimp occasionally needs reconnecting to the form in these cases.