Simple concept: Project Server - The Web site specified is already linked to another project. Select another Web site, and then try again

[Question]:
I have Project Server 2010. On the page /_layouts/PWA/Admin/ManageWSS.aspx I see none of the project workspaces are linked, a column Site Address is empty.

When I try to edit Site Address and add existed project workspace to one of the project, I have got an error: The Web site specified is already linked to another project. Select another Web site, and then try again.

How can I determine to which project the project workspace is linked? And why Site Address is empty?

[Answer]:

You can determine the linked project for workspace with 2 following steps:
1. Run powershell :

$web = get-spweb {workspace url} 
$web.AllProperties["MSPWAPROJUID"]

You will get Project Id to which the workspace is linked to.

2. Go to SQL Management studio and query:
SELECT *
  FROM [ProjectServer_Published].[dbo].[MSP_PROJECTS]
  where Proj_UID = {MSPWAPROJUID value}

That will bring to you project to which the workspace is linked.

And the second question: Why Site Address is empty on /_layouts/PWA/Admin/ManageWSS.aspx even though the workspace is linked to the project
I don't know the reason., but I know you can easily re-establish the value.