License expired warning in Parallels Plesk Panel

rror: The license key is invalid. In order to use the Parallels Plesk Panel, please obtain and install a new functioning license key.
Your license key has expired. To continue using your Parallels Plesk Panel you must purchase a non-expiring commercial license key.

Cause
The issue may occur if the system time is incorrect.

Resolution
On the same page where the warning is shown, check the "Key expiration date":
If the "Key expiration date" is in the future, check the system time by running the "date" command in the Linux shell or "date /T" in the Windows command line. If it is incorrect, change it to the correct one.
If the "Key expiration date" is in the past, it means that the key is, indeed, expired.