This is probably a problem with access rights. Note that on Windows even an administrator may be denied access to a file. In these cases the admin must take ownership of the file to access it.
The problem may be that access permissions for the old versions of the file are different from the current version. You can access the old version's permissions on the "Old versions" tab ("Properties" context menu); check that permissions are sufficient. Again, an admin may need to take ownership to access the data.
Finally a note: You do realize that shadow copies are not generally considered a backup, don't you? Among other problems, they may be deleted automatically when space runs low, and they do not protect from hardware problems (dead harddrive). Please make sure you have a proper backup as well.