Cannot create linked mailbox in a Resource forest (4286331)

MFG cannot mailbox enable the corresponding AD User object.

When running below cmdlet fromthe Exchange shell:

"Enable-MailBox -Identity “CN= TestLinkMailbox,OU=ContainerName,OU=Name,OU=Name,DC=Name,DC=Name,DC=com” -Database MDB_Name -DomainController “DomainControllerName.com” -LinkedDomainController DomainControllerName.com -LinkedMasterAccount Domain\AccountName -Confirm False -LinkedCredential (get-credential) ’."

It following error appears:

Failed when resolving the linked main account, and checking to see if it exists in a whole structure that is different from the Exchange hosting forest. The error message is: Active Directory error '0x51'
Checking the usability of the server 'dc1bs.lba.de': 'Active Directory response: The LDAP server is not available.'.
    + CategoryInfo: NotSpecified: (:) [], TaskInvalidOperationException
    + FullyQualifiedErrorId: [server = GRW20-PSH01, RequestId = b3c0a51f-63ab-4195-838d-7b58b85453a0, TimeStamp = February 23, 2017
   13:48:11] [FailureCategory = Cmdlet-TaskInvalidOperationException] 827F33F4
    + PSComputerName: grw20-psh01.zd.bund.de

WORKAROUND:

Ensure the Resource Forest can communicate with the User Forest.

The MFG Release Notes states that in the user forest, MFG requires an account with read permissions to the AD, such as a domain user. MFG doesn't make any changes to the user forest; it only performs searches.