Protected directory cannot be enabled: "columns siteId, path are not unique"

Symptoms

When trying to add a new customer and protected directory in Parallels Plesk, the protected directory cannot be enabled:

Error message : ProtDir_IIS::update() failed: Add Protected Directory failed: columns siteId, path are not unique (Error code 1)

Cause

The protected directory was not synchronized properly.

Resolution

Synchronize the protected directories database using the following command:

"%plesk_cli%\repair.exe"  --synchronize-protected-directories-storage