Mail chimp plug in error when using a 100% discount coupon on membership 2 pro

I have just tested the Coupon facility and applied a coupon

I then joined the site on a test account and entered the discount code

Page reloaded to show £2.99 discount and sign up which is great.

When I clicked sign up on the MLC site it took me to a clear white page saying:

Fatal error: Cannot redeclare class Mailchimp_Folders in /home/mylegal/public_html/wp-content/plugins/mailchimp-sync/mailchimp-api/Mailchimp/Folders.php on line 3

Then I pressed F5 and it took me back to the MLC page and the screen that appeared was the normal registration complete page which is correct

How can I stop the fatal error appearing? I am assuming that it has appeared because no fee was paid for the membership so Mail Chimp are not accepting the user as a new newsletter member???

The coupon worked perfectly and I am registered as a member having not had to pay.

I tried to use the coupon again and it came up with “No Coupons remaining for this code.”