Hi Colin,
While generating reports ADmanager Plus queries the first Domain Controller configured in Domain Settings-->Edit Domain Details. In order to make sure the report gets generated properly make sure Authentication details(Re enter the credentials even if it is already present) along with appropriate Domain Controllers are configured under Domain Settings-->Edit Domain Details.
Thanks & Regards
ADManager Plus Team
Toll Free:+1888-720-9500
Email: support@admanagerplus.com
ADManager Plus - Active Directory Management & Reporting Software