Ok, without knowing what the portal is I'm afraid its difficult to know what the trigger or the application launch is calling and how it interfaces with the client. As you say, with or without the Horizon client installed, the problem remains the same, therefore it is suggesting that perhaps an extension of the portal needs to be registered on the client device for it to know to pass the previously authenticated session via SAML to the Horizon broker and into the Horizon client on the device.
If you can find out who owns the portal and what it is, that might be the best next line of enquiry.
Sorry I can't be of much more help.
Thanks
Andrew
