I just installed Mailchimp's Ecommerce 360 plugin into my Zen Cart install and I suspect that there might be a conflict with the Newsletter Discount plugin.
After I installed the Mailchimp plugin, I didn't get any errors reported when I started up my store. What did happen, though, was that the Checkout page failed to completed load when I tried to Checkout. It stopped loading right when it was computing the price (and I assume the discounts). As soon as I removed the Mailchimp 360 plugin files all was well again.
I have sent in a report of this to the MailChimp customer service, but I thought you might like to know that this was my experience.
(And if I had to choose, I'd choose your Newsletter Discount -- thank you for the lovely plugin. )
Here is the link to their plugin: