Update 8/12/2015: I’ve determined that the problem is due to my attempts at creating a custom Windows.UI.Logon.pri file to display a logon screen background image. Neither the utility nor the PowerShell script referenced below create a Windows.UI.Logon.pri file that is acceptable to Windows. In the meantime, I’ve decided to just eliminate the “Hero” wallpaper in favor of a solid color.

I’ve encountered a problem with Windows 10 Enterprise with Cumulative Update for Windows 10: August 5, 2015 (KB3081424) applied, where the secure logon screen is completely black instead of displaying the Ctrl+Alt+Delete message, clock, background wallpaper, etc.

Prior to KB3081424, I experienced a different problem at the Ctrl+Alt+Delete screen, where the lock screen wallpaper configured via Group Policy was not applied and the background was instead a solid blue color.

In both cases, the behavior was/is only evident when the registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\DisableCAD is set to 0 (enforcing the Ctrl+Alt+Del screen). Deleting the DisableCAD value avoids the problem, however, it also removes the benefit of the secure logon screen.

When the computer is locked (for example, by hitting Ctrl+Alt+Del and selecting the Lock option) and the screen goes black, the following event is written to the Application event log:

Log Name:      Application
Source:        Application Error
Date:          8/7/2015 5:06:41 PM
Event ID:      1000
Task Category: (100)
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      mycomputer.mydomain.com
Description:
Faulting application name: LogonUI.exe, version: 10.0.10240.16384, time stamp: 0x559f398c
Faulting module name: Windows.UI.Xaml.dll, version: 10.0.10240.16412, time stamp: 0x55b9a054
Exception code: 0xc000027b
Fault offset: 0x0000000000494ab9
Faulting process id: 0x1fdc
Faulting application start time: 0x01d0d15d56f2c0bf
Faulting application path: C:\WINDOWS\system32\LogonUI.exe
Faulting module path: C:\Windows\System32\Windows.UI.Xaml.dll
Report Id: 351d24a4-a031-4135-b595-334baef2ba99
Faulting package full name: 
Faulting package-relative application ID: 

Watching the LogonUI.exe process with Sysinternals Procmon shows that the process crashes when the computer is locked, causing the screen to go black. Hitting the Ctrl+Alt+Del key combination at the black screen launches a new LogonUI.exe process, the wallpaper image loads, and I’m taken to the logon page where I can enter my username and password and log on.

The problem exists on physical hardware as well as in a VMware Workstation 11 virtual machine, so I’m confident that it is not a driver-related issue. The only other particular thing about my setup is that I’m generating my own Windows.UI.Logon.pri wallpaper file using the utility discussed at: http://www.askvg.com/how-to-change-or-disable-login-screen-background-image-in-windows-10/ (although revisiting this page now indicates that .pri file generated by the utility may be responsible for lock screen problems).

I’ll test with the default wallpaper image, and with my desired wallpaper image repacked using the PowerShell script at: http://pastebin.com/aMBrHRwd .

Even if the Windows.UI.Logon.pri is the cause of the LogonUI.exe crashes, a bad file seems like something that the LogonUI process should tolerate.

I posted the problem to Microsoft’s Windows Feedback on 8/10/2015. A Google search turned up a few other people who have had problems at the lock screen, and there are a number of reports of similar problems going back a few versions of Windows.