Resource/User Forest LDAP configuration.
When the network topology is a Resource/User forest, to retrieve the relevant information the connection string of the resource forest must be queried first. Thus it needs to be configured first in the LDAP settings in the Admin Portal.
The second connection string will be the User forest connection string.
Additionally, In the maintenance service the following feature must be enabled:
Settings → Maintenance Service → Active Directory section → Synchronize SIP addresses from AD → Yes.