Despite successfully signing into the IDP, the sign-in to the Veritas Alta Archiving portal returns the following error, indicating that the SSO Sign-in attempt failed.
https://personal.us3.archive.veritas.com/Views/Regular/LoginGateway.aspx?IsSSO=False&Response=7SjkYnDNhQ7V8BXsvUAfeA2

This error can be captured in Browser Network trace or in Web Debugging tools like Fiddler.
A mismatch with the username with which the user signed into his IDP, and the one set on the archive.
For example:
User signed into the IDP with username johndoe@democloud.com and the archive in Veritas Alta Archiving has username as john@democloud.com.
Match the username on the archive with the username with which the user signs into the IDP, and then attempt to sign in again. If the issue persists, contact Veritas Technical Support. for further assistance.