Fabien, I suspect that something is broken, not connected properly or there is a short somewhere.
If there is, it's far from obvious. The board and the code are behaving properly other than this oddity.
The N mini is somewhat simple: do you measure +5V *after* the N regulator, when the Vin is about 9v?
I agree, it's simple to work with, and yes, there's 5 volts after the voltage regulator when VIN is 9 volts. The servo that I am using requires a minimum of 4.8 volts.
Assuming that you've checked the soldering, could you post the schematic?
Yup, I checked every single solder join.
As for the schematics: unfortunately, I don't have the cycles to create detailed schematics right now, but I plan on doing it in the near future.
Thanks for your interest and your help