I've asked our development team to see if they can get in touch with MailChimp about the issue, since we've been getting a lot of back and forth between our mutual clients. Unfortunately, this isn't something we can fix or address or even assist with from this end. All our system does is send order data to an address that has been provided by MailChimp. If that address is incorrect or if they are unable to parse the data, we can't do anything about that from here.
All the order data is still being saved and the orders are being processed normally otherwise, so fortunately the buyer names and email addresses that aren't currently being recorded by MailChimp will still be saved in your transaction logs.