(250) List_MergeFieldRequired error

Hi All,

I’m using this plugin now for a while on different websites. We have installed a new website like 4 days ago, worked all fine in the beginning. But currently when I submit I get the following error in the log:
(250) List_MergeFieldRequired

What does it mean and how do I solve this problem?

https://www.ads-software.com/extend/plugins/mailchimp-widget/