ERROR: Contact Form 7 > Mailchimp API Error: Not Found. Resource Not Found.

Hello,
I have seen this issue in several threads but none was resolved. I have setup MC4WP to use within my CF7 form by passing mailchimp audience ID (as explained in this docs https://www.mc4wp.com/kb/different-lists-different-cf7-forms/):

<input type=”hidden” name=”mc4wp-subscribe” value=”1″ />
<input id=”mpListImport” name=”_mc4wp_lists[]” type=”hidden” value=”<valid-audience-id>” />

When testing this worked fine 1! time (I received the Double-Opt-In Confirmation Email and the Contact got successfully added to the audience). But then it stopped working and I keep getting the error in the logs:

[2020-02-20 13:04:55] ERROR: Contact Form 7 > Mailchimp API Error: Not Found. Resource Not Found. The requested resource could not be found.
Request: POST https://us15.api.mailchimp.com/3.0/lists//members – {“status”:”pending”,”email_address”:”taja******@xh******.com”,”interests”:{},”merge_fields”:{},”email_type”:”html”,”ip_signup”:”2.XXX.174.198″,”tags”:[]}
Response: 404 Not Found – {“type”:”https://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/&#8221;,”title”:”Resource Not Found”,”status”:404,”detail”:”The requested resource could not be found.”,”instance”:”b64e21ed-365c-4b5b-8c7e-14a6405a6180″}

I tried renewing the API key, renewing the list, adding explicit field naming (mc4wp-FNAME, etc.), testing with my Gmail-Account. But the error keeps on.

Can you please help me resolve this issue?