1. Type cmd in Start menu.
2. Right-click on the first result (cmd.exe) and click on Run as administrator (confirm by clicking Yes if required).
3. Type

 slmgr -rilc
 net stop sppsvc
 cd %windir%\ServiceProfiles\NetworkService\AppData\Roaming\Microsoft\SoftwareProtectionPlatform
 rename tokens.dat tokens.bak
 net start sppsvc

4. Restart your computer 2 times.


* An alternative might be to run "sfc /scannow" (without quotes)

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Credits to Opa (MDL).