Can't change permission on folder and says "Admin" name isn't a valid

Is "Admin" the actual admin user name? Unless you actually created an admin user account called "Admin," that will not work.

On the Mac, open System Preferences Accounts pane. The admin user (there may be more than one), is the user in the sidebar with "Admin" below the name. Enter that user name as the admin user, along with the password for that user, to authenticate.

So someone actually created an user account on that Mac named "Admin"... When you open System Preferences Accounts pane, in the sidebar, there is a user listed with the full name (or short name) that is precisely "Admin"?

The problem appears to be the user name, because the error says "The name you entered isn't valid."

Is there any other user account listed there that is admin? If so, try using that user name (assuming you know that user's password).

That might work. But you'll need to give authorization using an admin user name and password, which seems to be precisely the problem are having now (when you try to change permissions on that folder).

When you enter "Admin" as the user name, it may be case sensitive, so make sure you are entering it precisly as it appears in the Accounts pane.

Verify volume failed with error Could not unmount disk (-10000)

That could happen if there is another process that is presently accessing the volume. I don't think it related to the volume having data corruption. The fact that is it a "server" may be relevant.

In the time that you have had this problem, has it been shut down and restarted?

You can also try restarting it with the Shift key held down. That should do a Safe Mode startup, if an Xserve acts like a regular Mac.

http://support.apple.com/kb/HT1564

http://support.apple.com/kb/HT1455

The Safe Boot will do some maintenance and tests during startup, and start up with only essential proceses running. Try accessing that folder now. If you can access it, you should probably save it off on another volume, such as an external drive, so that you have a current backup.

Try running Repair Disk Permissions and Verify Disk again in Disk Utility. Repair Disk Permissions will probably give you a lot of messages; they are usually just "informational" so don't be concerned. However, if Verify Disk reports an error, that is a serious problem that needs to be addressed.

Edit: If the volume being tested is NOT the startup disk, then Repair Disk Permissions will be disabled. It only applies when there is a system installed on that volume.