You received a message saying that:
The syncing between CCB and Mailchimp has been interrupted/skipped for sync pair <CCB Group / Saved Search> and <Mailchimp Audience/group>.
Main Issue (Highly Probable)
The issue is likely with the current merge fields on your Mailchimp (MC) account.
You may be using a MC form connected to your group/audience with fields you marked as REQUIRED. E.g., food allergy. When CCBChimp syncs, MC rejects the entire sync of that profile if the required field is blank.
Suggestion: Marking those fields as not required might fix the issue. If you need to require specific merge fields, link the form to a different group that isn’t synced through CCBChimp.
Another possible issue is that the contacts have invalid merge field values set, but the error is only triggered WHEN we try to sync. So it works fine for you if you use MC only, but MC suddenly rejects that record as it sees an error in the value of the merge field.
Say you created a merge field for MOMS with options like YES or NO but you added (YES)—including parentheses—or something like that. Then that would cause an error.
Suggestion: Fixing those merge fields with the potential bad values should help.
Our best guidance
Up to now, we have seen that the issues are on the Mailchimp side. So, we recommend looking at the records which are failing and checking out the values in the merge fields.
You may wish to read this article from Mailchimp for more details about merge fields:
Another Mailchimp helpdesk article also gives some information about merge errors. It says:
MMERGE3 Must Be provided Error
When attempting to force sync your CCB groups or saved searches with Mailchimp Audiences, you may get an error message that states,"MMERGE3 must be provided - Please enter a value" All this error message means is that the MERGE3 field is set to "required" in Mailchimp, and must be unchecked /disabled.
Login to your account on http://mailchimp.com
Navigate to the Audience tab.
If you have more than one audience, click the Current audience drop-down and choose the one you want to work with.
Click the Manage Audience drop-down and choose Settings.
On the Settings page, click Audience fields and *|MERGE|* CCB groups or saved searches.
In the fields in the Default value column, type the default values for your merge CCB groups or saved searches.
Clear the checkbox "Required" for MERGE3 and click on the Save button.
Now attempt to Force the sync again in CCB.
Click here for more information: https://mailchimp.com/help/set-default-merge-values/