Active Directory Cross Forest Trust
Group Policy uses the information collected during preprocessing to apply settings to the computer or user. The Group Policy service cycles through each client-side extension, sharing the previous collected information. Each client-side extension then applies its specific policy settings to the computer or users. During this process, one or more client-side extensions may report problems when attempting to apply policy settings.
Enable cross-forest user Group Policy processing
Open the Group Policy Management Console (GPMC).
Create a new Group Policy object (GPO) or select an existing one.
Edit the GPO and enable the following policy setting: Allow Cross-Forest User Policy and Roaming User Profiles (located in Administrative Templates\System\Group Policy).
Log off and restart the computer.