When StoreFront configuration or any third-party Web Interface implementation entrusts the Delivery Controller or XML service to authenticate users in slow responding active directory environments, the performance of the Delivery Controller (XML service) might be affected. Due to which enumeration would start failing.

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

The slowness in the Active Directory environment should be fixed.

From the Citrix side, the issue has been fixed in CVAD 1912 Cu3 and CVAD 7.15 Cu7.

To enable the fix, add the following registry key:
 
Location: HKEY_LOCAL_MACHINE\Software\Citrix\DesktopServer
Or
Location: HKEY_LOCAL_MACHINE\Software\Policies\Citrix\DesktopServer
 
Name: DisableGetPasswordExpiryInfo
Type: DWORD
Value: 1 

[CVADHELP-15536]

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Problem Cause

The issue is caused by the slowness in the Active Directory environment causing delays in the XML service to complete the authentication requests. This eventually spikes up the resource consumption of Citrix Broker Service and make it unresponsive.

CVAD 1912 Cu3 https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/1912-ltsr/whats-new/cumulative-update-3/fixed-issues.html
CVAD 7.15 Cu7 https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-15-ltsr/whats-new/cumulative-update-7/fixed-issues.html