Active Roles Synchronization Service: Create Skype User LDAP server unavailable


I'm trying to create/enable a skype user with the Active Roles Synchronization Service workflow. But when i try to create the user, i recieve the error:

Message-"The LDAP server is unavailable." LdapError-"ServerDown" "FQND.OF.THE.DC":"389"

I can change the Domain controller with the paramater -Domaincontroller in the workflow. But i'll recieve still the same error.

The Domain controllers are reachable from the Sync Service server. 

Does someone has a clue?

Kind regards,