I am French to. I had the same problem. The NC stop brutaly after 4 days. I send the M40 to my reseller and after 3 weeks i received a new M40.
It work fine and i think the sound is better with the NC than the first on. My new M40 works fine since 2 weeks.
Perhaps they have problem with the first series (I bought the M40 as soon it was released in France).