Attempting to launch a XenDesktop 5.5 or 5.6 desktop through an NetScaler Access Gateway throws the error: “Unable to launch your application. Contact your help desk with the following information: Cannot connect to the Citrix XenApp server. Protocol Driver error”

Problem

You attempt to launch a XenDesktop 5.5 or 5.6 desktop through an NetScaler VPX Access Gateway but receive the following error message from the Citrix Receiver:

Unable to launch your application. Contact your help desk with the following information: Cannot connect to the Citrix XenApp server.
Protocol Driver error

Solution

While this error message can be caused by various reasons, the first to check should be the following KB:

Case Study: The Protocol Driver Error is displayed when starting an Application through Access Gateway Enterprise Edition in the ICA Proxy Mode
http://support.citrix.com/article/CTX125718

… to determine whether you have maxed out your concurrent user sessions configured on the Access Gateway as shown in the following:

Access –> Gateway –> Change authentication settings

Note that you can also check to see how many active user sessions there are by navigating to:

Access Gateway –> Active user sessions

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I’ve also come across the situation where the NetScaler had no active sessions but was still suing evaluation licenses so when I went ahead to put the permanent licenses on, the error went away.