Plesk is not accessible: The number of websites hosted on the server exceeds the limits defined by your license

Symptoms

• Plesk is not accessible, while an error that is similar to the following appears in the Plesk GUI:
  

  PLESK_ERROR: The license key is invalid. In order to use Plesk, please obtain and install a new valid license key. The amount of currently used resources exceeds the limits defined by your license. The number of websites hosted on the server exceeds the limits defined by your license. You have 11 sites hosted; your license allows hosting only 10 sites.

Cause

The number of websites hosted on this Plesk server exceeds the limits defined by the currently installed Plesk license, which is not allowed.

Resolution

1. Obtain a Plesk Web Host edition trial license and install it
2. Remove a domain from Plesk, so that you may bring down the number of domains as necessary
3. Go to Tools & Settings > Licensing Information and click Roll back key in order to switch from the trial license back to your regular license