This happens only when the NAS is connected to domain with multiple domains in the forest and after the upgrade to DSM version 7.1. The default Management Mode setting is “Trusted Domains” with all domains in the forest selected. To resolve the issue with the reports, change this setting to “Single Domain with OUs”.
Go to Control Panel -> Domain/LDAP and click the Edit button. From the drop-down, select “Single domain with OUs” and click the Save button
It’s strange this is not set by default, luckily it’s easy to resolve