Separate Displays
Most likely fix would be to adjust the Display ID in player options
Please see I added a new Player and it went straight to "Display is active and ready to start"? I'm using an imaged/cloned version of Windows and all my Players show up as the same display in the Xibo CMS?
WatchDog & Screensaver
In this case you might consider disabling watchdog
Please see The Windows Player automatically restarts itself / Windows Player Watchdog
Screensaver Background Issues; Temp Files, Not Refreshing
That’s interesting, do you have IE11 installed on this PC?
Xibo 1.7.8 does change the registry (browser emulation) during installation, but IE must be installed to make it work.
Please make sure it is installed and that you have correct entry in windows registry - Enabling HTML5 in the Windows .net Player
Is your background an image(if it is please make sure it’s .jpg) or just the default black background?