This says it is adding "cn=mygroup", and there is a constraint violation of some sort. You'd need to provide significantly more detail about your setup as you seem to have some set of overlays in use that you haven't disclosed.
This uses the memberOf overlay.
The entry it is having an issue adding is this one. I believe it is because those users do not yet exist, because syncrepl decides to try and sync this entry before the users.
dn: cn=mygroup,ou=Group,dc=hpc,dc=cam,dc=ac,dc=uk objectClass: groupOfNames objectClass: top cn: mygroup description: My Group member: uid=user1,ou=People,dc=domain,dc=com member: uid=user2,ou=People,dc=domain,dc=com member: uid=user3,ou=People,dc=domain,dc=com member: uid=user4,ou=People,dc=domain,dc=com member: uid=user5,ou=People,dc=domain,dc=com
Many thanks, Jonathan