Your seeing the same issue (or close to it) we are having in our enviornment. External connections are getting a Blank\Black screen when connecting via PCoIP. I would ask you if the issue is the same when you connect via the RDP protocol? If so then my quess you be that you are having the same issue we are having. I have been assured that VMware's developers are working on this 5.1.x issue in my ticket I have open with them but unfortunetly the only solution I have found to get my users back up has been to roll the View Agent back to 5.0. I was able to leave my servers and clients at 5.1.x.
The problem seems to reside in the Agent and how it tries to connect with the View Client. Our environment utilizes a VSS with PCoIP Gateway enabled. The firewalls on all ends were disabled and we still received the same result. What we found was that the View Agent was sending out a single packet directly to the IP of the machine running the View Client (basically the IP of your home machine). This packet did not travel along the PCoIP Gateway and would loop within our network going no where. At this point the Agent seemed to wait for either this packet to fail before moving on or something of that nature causing the PCoIP port to become tied up and unavailable. To test this our network team created a terminating point with the same IP of my home computer at which time I was then able to connect via PCoIP without issue. In short the issue seems to be a bug with 5.1.1 and 5.1.2 (tested in my environment at least). I know this information sounds crazy but it is what our research and the log files showed us.
I hope this information helps you out some.