With the website finished, I finally had time to continue programming and testing my LibreServo, but the joy was short-lived.
After soldering the three PCBs together, I proceed to power the LibreServo for the first time from a battery instead of the ST-link programmer ... and a white smoke preceded by sparks made an appearance. The STM32F302 microcontroller has been completely blown up, and there is a huge short between ground and the 3.3V line. After looking closely, I see that there are tin chips between the legs of my microcontroller, right between vdd and vss! Is it possible that when welding the three PCBs I was not careful enough and some "chips" has ended up where it should not?