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Since the application has been previously activated with the official key, it started giving such an error - in this case, try the following manipulations. Check if the date, time, and time zone are set correctly on your PC. The wrong date can break synchronization with the Microsoft server. Try updating your Microsoft Office application. Accept any up-to-date updates. You can accept updates from any Office application, for example Exel => File Menu => Account => Updates. Check if you're using proxy or VPN services. They may also be responsible for this error. If none of these changes will help, contact your activation key dealer. Or you can buy a new license key for your version of Microsoft Office.