This article describes how to deal with the Citrix VDI issue when using SSL VPN.
When the user is trying to access the Citrix VDI server with the clientless SSL VPP (web mode), the error: 'Unable to connect to the server. Contact your system admin with the following error: Protocol Driver error' appears.
Starting from FortiOS version 6.0 the java applet responsible for Citrix has been removed due to incompatibility as all the modern browsers have disabled the java applet.
The Citrix support requires the Citrix receiver to use the java applet, but as the java applet is removed, it will not work with web mode SSL VPN, so it is not supported in web mode.
v6.0 and above.
The workaround would be using tunnel mode.