I fought with this issue: I wasn't able to login in Plesk website management system, due to the fact that the license key XML got corrupted for a mysterious cause I didn't discovered.

I wasn't able to move up from this prompted error and to reach the Plesk splash page.

I googled a bit and I found these instructions:

Use the command in the command prompt:

"%plesk_bin%\keymng.exe" –get-key-number

key file on plesk windows is in:

C:\Program Files (x86)\Parallels\Plesk\admin\repository\keys\keyxxxxxx

when you change it, you have to delete the file:

C:\Program Files (x86)\Parallels\Plesk\admin\repository\registry.xml 

and it will be recreated after first accessing plesk interface.