Q: El Capitan login and lock screen issues/bug

Experiencing the exact issues since upgrade.

Do a backup, preferably 2 backups on 2 separate drives.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J.  When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder.  Select Library. Then go to Preferences/com.apple.finder.plist and com.apple.desktop.plist.  Move the .plists to your desktop.

Re-launch Finder by restarting the computer and test. If it works okay, delete the plists from the desktop.

If the same, return the .plists to where you got it  from, overwriting the newer one.

Thanks to leonie for some information contained in this.

Hello...

I try it but it´s not solve the problem , when I leve my computer for 30min when it´s wake shows the gry backroad same thinks it´s that possible that no body have solution for that, because I search on all google searching methods there is no one!

Saladdin

Works like a charm! Thank you!

Fix doesn't hold. Problem returns after reboot.

Maybe confirm that "Reduce Transparency" is unchecked in your Accessibility preferences?

System Preferences --> Accessibility --> Display --> Reduce Transparency (confirm this is unchecked)

I have the same problem and that box was already unticked, any other ideas?

I had a similar problem. If you have FileVault enabled, try disabling it. FileVault apparently messes with the transparency of the login screen for safety reasons and disables peripherals (such as an external display) until you log in. You can follow this guide (scroll down): Use FileVault to encrypt the startup disk on your Mac - Apple Support.