After a restart, CTRL+ALT+DELETE produces:

This workstation is in use and has been locked. The workstation can only be unlocked by \ or an administrator.
Press CTRL + ALT + DELETE to unlock this workstation.

If you try to unlock the computer, the following error message is displayed:

This workstation is locked. Only \ or an administrator can unlock this workstation.

I have observed this behavior when a non-existent screen saver or corrupted saver is configured at HKEY_USERS\.Default\Control Panel\Desktop.

To reslove this problem, when the logon screen appears, perform the following, in the exact order listed:

1. Clear the Username box.

2. Clear the Domain Name box.

3. Clear the Password box.

4. Press CTRL+ALT+DELETE.

5. Log on.

NOTE: If you logon before the screen saver starts, you do not receive the above message.

NOTE: Reconfigure a different locking screen saver (that does not consume server resources).