Make sure that the Backup Exec Server is not being accessed remotely (Not via RDP, VNC, Terminal Session, etc...) when attempting to install the license. Try logging in locally using a user with full administrator rights and attempt to add the DLO License/Option once again.  

Open the Registry Editor and check for and remove the following registry keys:

Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.

- HKLM\SOFTWARE\SYMANTEC\BACKUP EXEC FOR WINDOWS\BACKUP EXEC\13.0\DLO UPGRADE VERSION
- HKLM\SOFTWARE\SYMANTEC\BACKUP EXEC FOR WINDOWS\DLO
- HKLM\SOFTWARE\SYMANTEC\BACKUP EXEC FOR WINDOWS\DLO\3.0\ADMINCONSOLE\INSTALLPATH
- HKLM\SOFTWARE\SYMANTEC\BACKUP EXEC FOR WINDOWS\DLO\ADMINCONSOLE\INSTALLPATH

Under HKLM\SOFTWARE\SYMANTEC\BACKUP EXEC FOR WINDOWS\BACKUP EXEC\13.0\INSTALL
..Delete the DWORD "DLO Upgrade Version"

Check for the existence of the DLO folder and remove it:
..Program Files\Symantec\Backup Exec\DLO

Reboot the server and perform the installation again.

If the error still occurs, perform the steps listed in the TechNotes in the Related Articles Section below.