Ok, I just went in the settings menu, and choose to put my sweet N900 out of its misery and "push" / really just tapped back to factory settings and erase everything the user installed or registered... and there it goes.
Installation is on going, it just restarted with the CSSU installed.
Crossing my fingers and my penguin's too.
Everything's perfect
I just need to put my backups in place (nothing but conversations and calendar and a very small list of apps).
Thanks for the help, to all four of you !