I think that the root cause is that I did something slightly wrong with the Apache vhost.

The net effect is that
a) going to the frontend shows an ez page without css/js
b) going to the backend shows nothing at all - with a 'YUI is not defined' js error when I open the js console of the browser.

I think that A is acceptable, but B is not.
there should be a try/catch for errors loading YUI, and a small bit of js or html giving me some clue on how to start troubleshooting the issue.

PS: install instructions do not mention assetic dump. Is that correct ? In relation to the fact that a previous step does mention picking a (default?) SF env