Error 400 – Invalid resource (dropdown field)

Hi there,
we connect several forms to Mailchimp with this plugin and it has been working great. However, suddenly we get error logs on all submissions that contain a country drop-down (which worked before, too). We assume this is happening due to some update but we cannot find any way to fix it. From the log:

title: Invalid Resource
status: 400
detail: The resource submitted could not be validated. For field-specific details, see the ‘errors’ array.
errors[{“field”:”merge_fields.COUNTRY”,”message”:”Data did not match any of the schemas described in anyOf.”}]

Now this data was submitted:

Email: [removed]
Country (required): {“value”:[“Afghanistan”],”label”:”Country (required)”}

Whereas Afghanistan was just a test. Previously under the field “Country (required)” ONLY the item “Afghanistan” would be submitted without the other text like “{“value”:[” etc.

Any hint how to solve this would be greatly appreciated.