Is this a new install or an upgrade from 5.X that was previously working?
You've stated it does this for both internal and external traffic, that's a clue. Start by focusing on internal traffic, usually fewer firewall rules and easier to troubleshoot. Can you paste a screen of the secure gateway configuration in the View Admin manager for both connection servers? Improper configuration of the PCoIP secure gateway can also have this impact. Here's a sanitized version of mine.
Servers > Connection Servers > (Select One) > Edit
Servers > Security Servers > (Select One) > Edit
Unless tunneling is a requirement for internal connectivity, I usually untick these boxes for the connection servers servicing internal connections which results in a direct connect configuration, rather than tunnel.
The connection servers that are paired with external security servers, the configuration should appear as above.
This brings me to another question: how is the environment designed? Multiple connection servers and security servers or just one of each?
Also, what load balancing if any are you using?
Also, are you using PCoIP over (4172) or Blast (8443)? They are not the same. The OP is confusing as to which you're actually using.