TCP port 80 is unavailable or might be in use by another application. This port is required for BrokerAgent.exe. If this port is blocked or in use by another application, the VDA cannot register with the Site. If this happens, users might not be able to log on and access their applications and desktops.

To resolve this issue, verify that no other applications use the ports needed for the VDA. This could be done using the netstat -aon command in a command prompt window. 

Communication Ports Used by Citrix Technologies