I got the phone back... and I still have not put my sim cards in it. I switched it on to see if it worked and that's all.
The workshop did not disclose anything regarding what could have caused the phone to malfunction in the first place. The report says they had to change some parts on the motherboard. I sent them a message via their website, but I did not get any additional comment, actually they did not even reply.
So, in the end, I cannot even say anything more beside the fact that this was not a "structural damage", otherwise they would have refused to fix it for free. Therefore, the issue resulted from a hardware or software issue independent from my use, and could potentially happen to any other user...
My impression is that phones will always remain black boxes, because there is absolutely nothing common people can do about them when they stop functioning after their warranty period is over.
As far as I am concerned, I will just wait that I have a massive issue with my current phone to use this fairphone again. I am still very much disappointed with my experience and totally disillusioned with the great promises of that company.
Thank you for following that thread and good luck with your phone... that's all we can hope to have.