Microsoft rolls out Windows 10 build 21387 to Dev Channel Insiders

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It's been some time since Microsoft has released a new Windows 10 build on its typical Wednesday schedule, and if you were hoping this would be the week we go back to normal, it isn't. Microsoft has released another Friday build, this time being number 21387, and just like the past few releases, this one comes from the co_release branch, not rs_prerelease.

While we still don't know whether these builds will be part of the next feature update for Windows 10, the fact that they're coming from a release branch does suggest we may be getting closer to a stable build. In fact, there hasn't been a whole lot that's new in each build, and that's the case this week, too. This build only comes with minor changes, such as temporarily disabling Eco mode in Task Manager:

  • The Internet Explorer 11 desktop application is now retired as of this Insider Preview build. For additional details see
  • We are turning off the “Eco mode” feature in Task Manager in order to refine the experience and address several issues thanks to Windows Insider feedback.

The focus has been on fixes for the past few builds, and we have a few more with this week's build:

  • We fixed an issue in the previous flight where during the upgrade and in the welcome screens displayed after first sign in, the text was unexpectedly using the Times New Roman font.
  • We fixed an issue resulting in some Insiders seeing errors when trying to launch Notepad via the Run dialog.
  • We fixed an issue for WSL users, preventing some Linux GUI apps from launching correctly via the shortcut that is automatically added to Start menu.
  • We fixed an issue resulting in the reset or change PIN workflows not working.
  • We fixed an issue where for some Insiders, night light was immediately turning off after being enabled.
  • We fixed an issue in recent flights where you might unexpectedly get a “We can’t find an audio device” error when trying to play audio from a connected Bluetooth speaker.
  • We fixed an issue where explorer.exe might crash after repeated use of the touchpad gesture for switching Virtual Desktops.
  • We fixed two issues impacting the ability to print using USB printers.
  • We fixed an issue resulting in the Windows Update page potentially becoming unresponsive after clicking Pause Updates.
  • We fixed an issue causing some devices to fail with error code 0xc1900101. While this fix will resolve the problem on some devices, there is a second issue causing the same error code (noted below). We’re working on a fix for this issue as well.
  • We fixed an issue where some devices were re-offered the .NET update after it is installed. After taking this build you should no longer be reoffered the .NET update. If you experience and issue getting Build 21387 because of a pending .NET update, you can work around the issue by pausing then un-pausing updates.
  • We fixed an issue where some devices would crash with a DPC_WATCHDOG_VIOLATION error when using OneDrive Personal Vault.
  • We fixed a ctfmon.exe crash when using ATOK (a third-party IME) if reconversion was triggered in some win32 apps.

Despite those fixes, there are still some known issues in this build. Microsoft did have a Bug Bash over the past week, so we may see more fixes being added in over time. Here's what you still need to be aware of:

  • The Windows Camera App currently does not respect the default brightness setting set via the new Camera Settings page.
  • We’re investigating an issue where elements of Search (including the search box in File Explorer) are no longer displaying correctly in dark theme.
  • [News and interests] We’re investigating an issue where the flyout may occasionally flash in the top left corner of your screen after clicking the button on your taskbar.
  • We’re working on a fix for a second issue causing some devices to fail when updating to this build with error code 0xc1900101. When the update fails, it will roll back to the previous build. If you are encountering this issue, we recommend pausing updates until we release a build with the fix.

If you're not an Insider, or not receiving Insider builds right now, Microsoft rolled out the May 2021 Update earlier this week, though it doesn't include much that's new. It's still not known when we can expect the new features that have been showing up in the Dev channel will be available, but there should be another feature update to Windows 10 in the second half of the year.

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