1.netstat -ano | find "80" >> port 80 not being associated with brokeragent for IPV4.
2.VDA event log: Event ID 1023 Source: Citrix Desktop Service VDA failing to register with the error stating "The registration was refused due to AgentNotContactable".
3.DDC event log: Event ID 1039 There was no endpoint listening at http://VDA FQDN:80/CITRIX/VIRTAULDESKTOPAGENT/IQUERYAGENT that could accept the message.

1.Uninstall IIS on VDA
2.Delete the "::" in HTTP.sys settings.
  delete iplisten ipaddress=::

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Problem Cause

DDC -> VDA 80 port register traffic rejection, as no endpoint listening at http://VDA FQDN/Citrix/VirtualDesktopAgent/IQueryAgent could accept the message.
Two settings modified by users have caused this failure:
- The installation of IIS on Windows 10 desktop.
- HTTP.sys on Windows 10 desktop has been mistakenly changed to "::" which excludes IPV4 address and includes only IPv6 address.

 

https://superuser.com/questions/352017/pid4-using-port-80
https://docs.microsoft.com/en-us/windows-server/networking/technologies/netsh/netsh-http#delete-iplisten