Upgrade from Windows 10 Home to Pro fails with error code 0xc004f050

I have a PC running Windows 10 Home (valid license and activated). Now I bought a Windows 10 Pro Upgrade license (OEM) to upgrade that PC to Windows 10 Professional. If I enter the license key in the system panel to upgrade, this fails with this error code:

0xc004f050

I bought the license for a reasonable price from a trustworthy vendor, so I believe that the license key is valid. How can I activate/upgrade?

Just restart your PC after trying to enter your upgrade key. By this, Windows would magically update some files. After the restart, you will have an unactivated Windows Pro, just enter your license key again to activate your Pro version!