Hello again,
The problem still remains after all these months.
My current solution is to access a remote CentOS 7 machine through VNC
and use Horizon Client from there.
Any idea why this might be happening?
I don't know the history of your issue, but the first two thoughts that come to my mind:
1) Try a different Desktop Environment (e.g xfce4 instead of Gnome, etc)
2) Try the HTML interface (assuming it's available) instead of the client
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