XSI.exe always takes Advanced licenses
IssueWhen you start Softimage, XSI.exe does not use Softimage licenses, it always takes Softimage Advanced licenses. If you have only Softimage licenses, Softimage won't start.
Reinstall Softimage, and enter the product key 591B1 during installation. This will install the Network license version of Softimage. (You could also use 590B1, but then you have to choose Network License, not Standalone or Multi-Seat Standalone, as the licensing method.)
This problem happens with silent installs that use the wrong product key (for example, 590A1, which is the product key for Softimage 7.5).