I submitted a request and waited a while.
They told me to open an account on the return website and request a return.
(I backed up my contacts and sms, then spent some time looking for a replacement phone before I did it).
After a day or two, it was approved.
I chose a pick up date (different days available, but only “between 12:00 and 8:00 PM”).
The DHL guy picked it up around 1 PM (so I didn’t have to wait all day ).
I put the phone+battery+back cover in the original packaging, and covered the previous shipping tag with the new one.
I am now waiting for the phone to arrive to them, then to get their assessment (can they repair/replace the phone? do I have to pay, and if so, how much?).
Since the first answer to the request I submitted, it all went well and fast
Edit (2016-09-29) : I’ve got an email today: “We have just created a new order for you […] will bypass the regular queue and get shipped as soon as possible”. Hooray!