After upgrading Windows 2016 VDAs from 1912 LTSR CU9 to VDA 2203 LTSR CU 5 users started experiencing long logon times. The logon seems to get stuck in a black screen. Black screen is also observed when connecting over RDP to the machine. 

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Problem due to Microsoft OS UAC settings.

Creating the following registry key and restarting Appreadiness Service resolved the issue.

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HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System

Name: EnableLUA 
Type:   REG_DWORD
Value:  0

Logon process gets stuck in a black screen in both RDP and ICA Sessions.