The Citrix Native Access Module (NAM) in some circumstances can throw the error message Protocol driver error.
If you encounter this during the initialization of the citrix NAM (either Citrix farm or direct) the reason could either be that a) an internal firewall is still blocking access to port 1494 or b) routing is not setup correctly.
Case b) mainly occurs when OnDemand Tunnel is in use and the internal routers and firewalls are not set up correctly. If the firewalls are configured correctly and you still get the error, try without OnDemand Tunnel and just use the web proxy agent or OnDemand proxy.
If it then works, you definitely have a routing problem. Some customers will see -110 errors in the network audit log.
With 8.7, it can happen that the users have access to browse the farm but not to access the Citrix server running the published application. This is due to missing resource definitions in the AMC and and/or a lack of access rights to those resources for the user.