Just a brief blog post about a rare Windows issue. Some users are facing a strange problem: The UAC prompt won't be shown – it opens invisible in background.
I came across this issue due to a post in German Microsoft Answers forum, where a user complains about his effect under Windows 10 and Windows Server 2012 R2. After searching the web, I found a few hits here, here, and here (not available). All users describing the same effect: Invoking a task that requires administrative privileges, the UAC prompt won't be shown. The hear just the sound notification, but wasn't able to confirm the UAC (or confirm it blind via keyboard shortcuts).
In this old Windows 7 forum post it seems, that an old version of Microsoft Security Essentials (MSE) caused this issue. But in Windows 10, MSE isn't available – and Windows Defender can't be uninstalled. I guess, it could have something to do with graphics card or graphics driver. But it seems that the following workaround helps:
1. Opens Control Panel (Windows+X) and go to User Accounts and Family Safety > User Accounts and click to Change User Account Control settings.
2. Move the slider from the 3rd position down to the 2nd position Notify me only when apps try to make changes to my computer (do not dim my desktop).
Maybe it helps, if you are affected by this behavior.
Windows 10 Wiki
Check and repair Windows system files and component store
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