It looks like that field must be of type "number" and be limited to one value like you see in the image above. The "number of values" fieldset should be on the content type's manage fields page, e.g. "admin/structure/types/manage/<content-type>/fields/<field-of-type-mailchimp-subscription>".
I would check the manage field page to make sure it is set to one for the number of values allowed.
Wow, that sounds awful...
So maybe it's not even related to other modules, but some... cache problem somewhere? Be it browser or server-side? A Heisenbug.
Comments
The specific error you are seeing comes from https://github.com/backdrop-contrib/mailchimp/blob/1b970f66c112e88d2ccfe....
It looks like that field must be of type "number" and be limited to one value like you see in the image above. The "number of values" fieldset should be on the content type's manage fields page, e.g. "admin/structure/types/manage/<content-type>/fields/<field-of-type-mailchimp-subscription>".
I would check the manage field page to make sure it is set to one for the number of values allowed.