In Presto 2.5+ Presto authenticates users via the Presto Agent Service running on the host machine where the actual service is located.  

In the context of a multi print server environment authentication would occur on a secondary server, not where the main instance where Presto Server is located.  Where applicable, the Presto Agent Service must be set to run as a domain administrator for authentication to occur.

Microsoft KB: Configuring how a service is started.

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