I came across an issue in my lab environment where the screen will go black while launching a session on Windows Server 2016. This is with XenApp/XenDesktop 7.15 LTSR
The following registry entry: DisableLogonUISuppression (D WORD Value 0) did not resolve the issue as stated in the following articles:
https://support.microsoft.com/en-us/help/4034661/windows-10-update-kb4034661 and https://support.citrix.com/article/CTX225819
Ultimatly after some trial and error the deletion of all subkeys from below registry entries resolved it:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Connectivity
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration