Did you receive an email that looks something like this?
This likely occurs when the synced MailChimp lists are manually removed from your MailChimp account. If you or no one made manual changes to your MailChimp account, this is may have been caused by something else that shows a mismatch in your sync settings and your account.
To prevent any issues (e.g. overbilling), we require you to confirm and resave your synced settings.
Here's a step by step (quick) guide on fixing this issue
2. Click on Sync
3. Click on CCB to MC main tab
4. Click on Fix Now
5. Click on Select Audience
6. Click on the "replacement audience"
7. Click on Select MC Groups
8. Click on the "replacement group"
9. Click on Save
Don't forget to click on SYNC NOW to update everything or wait until overnight syncing happens.