This issue affects XenApp 7.5; XenDesktop 7.1 and 7.5 VDA Core Services for Windows Server OS (64-bit). After launching a published desktop or application, the screen may be completely black, or it may freeze.
Since this issues affects the default graphics mode, I'm scratching my head, amazed that Citrix doesn't include this fix in the ISO download of XenDesktop/XenApp 7.x. To make matters worse, you can't just visit the Citrix site and download this hotfix, you have to open a support ticket and they must email you a download link.
See CTX139901 for more info.