When registering UbuntuSrv.xml
The UbuntuSrv.xml you are trying to register has already been linked.
Go to \RadarOpus\Data\Database and delete the file.
Insert the RadarOpus installation DVD, go to to \Others\Platforms and copy the UbuntuSrv.xml
Paste it in \RadarOpus\Data\Database
Re-register the database:
"C:\Program Files\VirtualBox\VBoxManage.exe" registervm C:\RadarOpus\Data\Database\UbuntuSrv.xml
If it is necessary to remove the hard disk:
First remove Ubuntu.Srv (select and choose Discard)
In the VBox main menu -> File
Media Manager
Choose the hard disk to remove and click on the remove icon above
Keep the hard disk, do not delete, when asked