The problem was keyboard configuration.
For some reason, the greeter has been set to US-English keyboard (instead of French) when the console and once logged in Freya is still with the French Keyboard. As the keys are not mapped the same way, I was having an authentication failure.
This was clear when looking at the file:
I haven't managed to get the French keyboard back (for Greeter) yet but at least I can work around it:
- I can use the on-screen keyboard (not cool as anybody can see what I am typing ... but it works)
- I can type the password on the keyboard knowing the US keyboard map (if there's no punctuation signs that's doable).
Hopefully I will manage to get the French keyboard back for the Greeter.