After upgrading a client site to Sage CRM 2017 R2 when sending a large group of contacts .e.g 8900 records from SageCRM to Mailchimp the following error message is received

The group was not sent to MailChimp. Remove all bounced and unsubscribed email addresses from the group and resend it

The Group is a dynamic person group which is added to on a regular basis with new subscribers and has all emarketing opt outs removed in the search criteria and all contacts have an email address as another criteria of the group.

We can push smaller groups over to mailchimp of between 1 and 20 users with no issues, We have recreate the group again in SageCRM and still the same issue.

The mailchimp account has is a paid for 10,800 subscriber account and auto updates if the limit is reached.

Tomcat seems to be running correctly as dashboards are working correctly, report run and export to CSV and groups export with no issue.

Has anyone seen this error in the 2017 R2 release when using the mailchimp integration or aware of any logic changes when a group is now passed to mailchimp from SageCRM as this group pushed across a week ago before the upgrade?