Windows 10 Insider Preview 21387.1 (co_release) 210518-1436

Good Morning, friends! Microsoft has released a new last week for Windows 10 pre-build number 21387 for Windows Insider program members using the Dev channel.

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Windows 10 Insider Preview 21387.1 co release 2
Full build number: Windows 10 Insider Preview 21387.1 (co_release) 210518-1436

Recently, the branch name changed to CO_RELEASE. As in October, when insiders started getting builds from the FE_RELEASE branch, the company is practicing the ability to change the branch that is considered an active development branch. Since engineers work in different branches within the development cycle, it may be necessary to move insiders between these branches. The company treats these builds in the same way as the builds from the RS_PRERELEASE branch.

We remind you that the Dev channel releases a build that can contain quite unpleasant errors. If you need more stability, we recommend switching to the Beta or Release Preview channels.

Changes and improvements

  • Starting with this build, the Internet Explorer 11 desktop application will not work. More details on this can be found on the official blog.
  • The “Eco-mode” feature in the “Task Manager” is disabled, as the company is working on improving the work and fixing some problems.


  • Fixed an issue that occurred in a previous build where the Times New Roman font was unexpectedly used for text during the update process and on the welcome screens displayed when you first login after the update.
  • Fixed an issue that caused some insiders to error when trying to launch Notepad using the Run dialog box.
  • Fixed an issue with WSL that caused some Linux GUI applications to improperly set up using an auto-add shortcut in the start menu.
  • Fixed an issue where demand scenarios or PIN code changes didn’t work.
  • Fixed an issue that caused some insiders to disable the “Night Light” feature just after turning it on.
  • Fixed an issue seen in previous builds where users could unexpectedly get the “Can’t find a sound device” error when trying to play sound on a connected Bluetooth speaker.
  • Fixed an issue that caused explorer.exe to crash after repeated use of the touchpad action to change visible desks.
  • Fixed two issues affecting print capacity using USB printers.
  • Fixed an issue where the Windows Update page could become unresponsive after clicking on the “Pause Updates” button.
  • Fixed an issue where some devices are experiencing interruptions with error code 0xc1900101. This update will fix the issue on some devices, however, there is a second issue that causes an error with the same code (see below). Engineers are working on fixing the problem.
  • Fixed an issue where the update was repeatedly offered on some devices .NET after installing it. When you upgrade to this build, you will no longer receive a repeated .NET update. If you have trouble finding build 21387 because of a pending update .NET, then you can work around this problem by pausing and then resuming receiving updates.
  • Fixed an issue that caused GSOD to occur on other devices with DPC_WATCHDOG_VIOLATION error when using OneDrive Personal Vault.
  • Fixed a crash ctfmon.exe, which occurred when using ATOK (a third-party IME), if some Win32 applications were running a reconversion.

Known issues

  • The Camera app does not currently take into account the default brightness setting set on the new page in Settings.
  • We are investigating a problem that causes Windows Search elements (including the search field in the “Explorer”) to display incorrectly in a dark theme.
  • [News and Interests] We are investigating an issue where a pop-up window may blink in the upper-left corner of the screen after clicking on a button on the taskbar.
  • Engineers are working to fix the second issue, which crashes when trying to upgrade to this build with error code 0xc1900101. If the upgrade fails, the system will automatically roll back to the previous build. Users who encounter this issue are advised to suspend receiving updates until a patch build is released.

I wish you a successful update!

By Techfeeddata

I am Sanjit Gupta. I have completed my BMS then MMS both in marketing. I even did a diploma in computer software and Digital Marketing.

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