registry.xml is a file that contains information about Plesk licenses deployed on a server. When this file becomes corrupted, you may see the following error message in your control panel:
Error: The license key is invalid. In order to use the Panel, you need to obtain and install a new valid license key. The amount of currently used resources overrides the limits defined by your license. The number of websites hosted on the server exceeds the limits defined by your license. You have X sites hosted; your license allows hosting only X sites. The number of served customer accounts exceeds the limits defined by your license. You have X customer accounts currently served; your license allows serving only X customer accounts.
To fix the registry.xml corruption, simply remove the damaged file. You can find the file in one of the following folders, depending on your OS:
After the file is removed, log in to Plesk Panel. The registry.xml file will be recreated.