This situation occurred after the upgrade from MOSS 2007 to MSS 2010. When adding a document, or listing a document owner, the user ID of the author of the document does not appear. Typically a list item will read "Added By domain\username". In this case it read "Added by [blank]. Permissions within the document library and across the SharePoint Farm are still enforced, and there appears to be no loss in functionality, other than the User ID is simply not displayed. The picture below is from a site that had not had the visual upgrade applied.
To correct this, I logged in as an Administrator and added the user account (domain\user_acct) to the Site Collections Administrators group in SharePoint. This was in
Site Home -> Site Settings -> Site Collection Administrators
and added the user account temporarily to the Site Collection Admin group. I resolved the name to be sure it was spelled correctly, and then clicked OK.
The 'modified by' field / attribute now displays properly.
I then removed the user account from the Site Collections Administrators group, and the 'modified by' attribute continued to display the user name correctly.
I still don’t know what caused this, but at least the problem has been corrected.
