hey! I've been having the same issues... deleted the block, added it again and re-connected Mailchimp -- and still getting form submission errors. I've been manually taking the email address, putting them into a google spreadsheet, downloading a .csv file - and importing into Mailchimp. Certainly not ideal or streamlined!
UPDATED: I found this in another thread -- and connecting my Squarespace site to Mailchimp in Mailchimp's connected sites - was probably the issue!!
Here's from the other thread: You should connect your site with Mailchimp. Try the steps on this page... scroll down to "connect Squarespace" and add the code, then check the connection. (Shout out to Belinda who shared this in a Facebook group!)