Hello,
It might be a good idea to update to 1.7.7 CMS and clients.
Do you have any 3rd party software installed on your PCs that could potentially cause problems?
So, does it happens randomly or did it stop?
As always with Windows clients it might be also a good idea to:
Install IE 11 and make following registry change (browser emulation) - Enabling HTML5 in the Windows .net Player
Also, especially when you upgrade to 1.7.7, it would be highly recommended to have fully qualified path to your CMS library in CMS settings.
More information on it under ‘Library location’ section of this post https://community.xibo.org.uk/t/xibo-cms-post-installation-setup-guide/522