The email tracking history records where unable to be added from Campaign Monitor. This is typically due to custom code, processes etc that are preventing the email tracking history record (i.e. Campaign_Activity__c) being inserted into Salesforce. When the records are inserted, they are typically added with a transaction size of 1000 records. If you have a trigger that is linked to the Campaign_Activity__c object, then it must be able to cater with these volumes.Check the associated log details in Maintenance for details on which custom trigger/process etc in Salesforce is causing the issue. Disable that custom trigger/process and then perform a Full sync (to ensure the email tracking history is correctly updated).
To see all maintenance codes please click here.