Most Softimage customers have network licenses. Unfortunately, by default Softimage installs in Standalone license mode, and often people get stuck trying to activate their licenses at startup, because they get this error:

The serial number you entered is not valid for the product you are trying to activate. Try again. (Error 123)

Activation at startup is for standalone licenses only. Error 123 almost always means that you have a network license, not a standalone license. And with a network license, you have to:

• Install the Autodesk Network License Manager (LMTOOLS).
• Go to autodesk.com/register to download your license file.
• Install your license in LMTOOLS.
• Switch Softimage to Network licensing mode.

Here’s a video walkthrough. Unfortunately I forgot explain how to install LMTOOLS (run the Setup and choose Install Tools & Utilities). I forgot because I had to re-record the video (the first time the volume on my microphone was way too low).