MailChimp integration plugin API key not working

Because you're ignoring our other post https://premium.wpmudev.org/forums/topic/we-are-trying-to-use-the-wpmu-mailchimp-integration-plugin?replies=6#post-829154 i have to open a new ticket.

Please do answer this question. We're paying for WPMU and our customer really needs his website up und running!

problem:
We are trying to use the WPMU MailChimp Integration plugin:
https://premium.wpmudev.org/project/mailchimp-newsletter-integration/

When setting it up, we're inserting our API key in the field "MailChimp API Key" in the plugin's settings.
After updating we're receiving the following error message:
"Failed - Please check your key and try again."

We are using another 3rd party plugin for MailChimp in the same Wordpress installation:
https://wordpress.org/plugins/autochimp/

Here we're using the same API key (we only have one MailChimp API key) and it's working.

What are we doing wrong?
Maybe you've got some advice how we can debug that problem?

Best regards,
Andreas