Citrix Director throws error "The system is currently unavailable. Please try again later or contact your administrator".

Scenario 1: If Director server and admin users are not from same domain (Director is from forest A, admin users from forest B). 
Solution : Update the DNS suffix on the affected server.
https://support.citrix.com/article/CTX237695

Scenario 2: Check if there is stale entry of Controller by executing Powershell command 'Get-BrokerController'. 
Solution : Follow article https://support.citrix.com/article/CTX139505 to remove the controller.

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Problem Cause

1. DNS suffix on the affected server may not be correct in case of cross forest environment if Trust Type between both forest is External.
2. XenDesktop controller hostname has been renamed.