With certain programs (Adobe Reader, Visual Studio, ...) a black box is always displayed next to the cursor in XenDesktop VMs.
This only happens when a connection is made to an IGEL device and disappears when a Windows device is connected. The box is only visible directly on the client (not via VNC).
The problem occurs on the UD7 as well as on Intel NUCs.
In the IGEL Setup disable the parameter Disable RENDER Extension (for ICA only) under System > Registry > ica.wfclient.disablexrender:
See also at Citrix: https://support.citrix.com/article/CTX212013