When Free/Busy information is requested for an alias domain, or when the hierarchy of a certain root dn is not the standard expansion and implosion, DirectoryLDAP will not be able to resolve, for example:
@alias.tld => dc=parent,dc=tld
unless a `base_dn` is configured, but such setting is only used for / available to the one directory hierarchy and as such is not suitable for hosted deployments.