FIX: Citrix Presentation Server license acquisition error (500): Unable to acquire a license from server (<servername>). Please contact your system administrator or open Event Viewer for more details.

Scenario: 

You receive a Citrix License Acquisition Error (-25) or “Citrix Presentation Server license acquisition error (-500): Unable to acquire a license from <server name> server. Please contact your system administrator or open Event Viewer for more details.” when logging on to the server.

This issues starts all of sudden on a well working XenApp server and you haven’t done any changes to the environment.

Fix:

If this issue indeed started all of a sudden and you think the license server is all doing good and XenApp server is able to contact the license server then, most probable fix is to reboot your XenApp server.