400: your merge fields were invalid

Getting email alerts:

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The optin campaign “Subscribe For Latest Updates” is failing to convert leads due to the following error “400: Your merge fields were invalid.”.
Use the coupon MOSAVE20 on checkout to save 20% off MailOptin premium.
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Really?
a). no additional information to help troubleshoot this, and
b). shameless plug to buy the premium version because the simple setup of the free version is not working as advertised… yikes.

Please advise.