The 5.1 upgrade has three flavors: Cubase Score, Cubase VST, and Cubase VST32. If you accidentally downloaded the incorrect version, it will change your Cubase to this version, but then will search for the corresponding dongle (which is different for all versions.) You will need to uninstall Cubase from the Control Panel - Add/Remove Programs. Choose Custom Uninstall, and Select All - Next until it finishes. Reinstall from CD-ROM, and download the correct version, and it should work fine.