The black screen is a connection issue (firewall, routing, configuration, etc...). Are you currently load balancing the UAG? Persistence needs to be maintained during the primary and secondary horizon protocols from the endpoint to the UAG.

The UAG implement the HTTP(S) Secure Tunnel, BSG (Blast Secure Gateway) and PSG (PCoIP Secure Gateway). You must disable the HTTP(S)/BSG/PSG on the connection servers.