Microsoft fixed a bug with the "deactivation" of licenses for Windows 10

On November 8, 2018, many users of Windows 10 Pro began reporting about the same problem : their licensed OS unexpectedly began to show no-activation messages suggesting that they install Windows 10 Home or buy a “genuine copy of Windows”.

Some started a real panic: the thread on Reddit has more than 300 posts. People worry that Windows is "deactivated". Some called Microsoft support, where they were advised to buy a new license or somehow get a new key for the product. Individual users say they did it.
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It turned out that this is just a bug on Microsoft activation servers.

In the evening, Microsoft made an official statement that it was working to fix the bug. Later a more detailed explanation appeared :

We are aware of reports that some users of Windows 10 Pro and Windows 10 Enterprise machines do not see their licenses as activated. Users may receive the following notifications: “Error: you are using Windows 10 Pro, but you have a valid digital license for Windows 10 Home” or one of the following error codes: 0X803F8001, 0xC004C003.

We are actively working to solve this problem within 24 hours. You can still use your computer; however, the watermark will be displayed until activation is restored. Thank you for your patience.

We will update this post with additional information as needed after the activation is restored.

Some Windows users bought a new license. Others followed the instructions and installed Windows 10 Home instead of Windows 10 Pro. The rest were waiting for Microsoft to roll out a patch with bug fixes. At this moment, the bug on the activation servers seems to be fixed and the license can be restored in the menu Settings - Update & Security - Activation - Troubleshoot - Activation Troubleshooter .