Hi,

I have a Dell U2715H monitor and I use it via Displayport-Displayport with my Evga GTX 1080 SC at 1440p and 60Hz.
Sometimes, on rare occassions, the monitor will show black horizontal lines on the whole screen - it looks like this: 

This happens EXCLUSIVELY when turning on the pc at the Win10 login screen. It never happens during regular usage. If I enter my password the screen turns black for a second and I get to see my desktop as usual without the black-line-issue. If I turn the monitor off and back on the issue is gone as well.

What could be causing this? It's also freaking hard to test and look for the error causing this behaviour since it's happening so rarely.

relevance
I remember having similar issues on Windows 8.1 with blur images like that. it was the FXAA causing on Nvidia control panels on global settings. putting back to "application control" solved the problem for me. not sure if this is related issue, but please check and see.

The global FXAA can only be set to on or off. There is another FXAA option which is set to "app control".

Make sure your dp cable is vesa certified. You can find vesa certified dp cables here.