Hi Luis,
Good suggestions. I've created a new admin account, added some photos to Photos, added some music to iTunes, and then let the boot of FCPX rip. It ran a large number of scripts at it's initiation and displayed a splash screen, which it didn't do on my account on a full new install.
However, again the "Photos" and "Music and Sound" libraries remain empty. I also added a project and a clip and do a basic workflow with a transition, but the two views remain empty.
I repeated the same with a user that was existing, but had never used FCPX. Same results as with a fully new user/account.
So the conclusion is that it's NOT specific to an account. I will now try Onyx.