A problem occurred whilst trying add or update Salesforce Campaign Member records that are linked to a Campaign Monitor  campaign. You many notice missing update to Campaign Member statuses or missing new Campaign Members that have signed up via Campaign Monitor.Typically this is due to conflicting custom code (process builder flows, triggers, validation rules etc). The associated log message to this error should provide details on the conflicting custom code/configuration. Remove the conflict and then perform a Full sync to recover from missing member records or member status updates.

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