Cookie Error on Citrix XenApp

A user reported that the following error while visiting a website on a Citrix XenApp server:

I tried adding the site to the Trusted Sites List and adding the url to the Per Site Privacy list:

But this didn’t work, but I noticed that the site was “flickering” a lot so I suspected that HDX Flash Acceleration was the problem.

I imported the adm file for HDX (HdxFlash-Server.adm) into a GPO and added the site to the Per-URL-blacklist:

That fixed the problem!