In the event Log's search for Audit Failure and see if there is a message looking like "Unable to launch from Pool XXXX for user domain\username: No co-management availability for protocol PCoIP"

Next step, look at your connection servers and take a look at your CPU usage.

Lastly, add a few more vCPU to your Connection Server's.  This worked for me to alleviate 99% of that issue.  Though it still happen's from time to time, i know there is an open Engineering project internal to VMware on the issue.