Hi Loic,
Which paparazzi_version are you using? And are you trying to use an aspirin2.1 with an external MS5611 or do you really have one with the baro mounted on aspirin imu (v2.2)?
The <configure name="LISA_M_BARO" value="BARO_MS5611_SPI"/> currently only works for the rotorcraft firmware, it has no effect for fixedwing.
So you shouldn't even have any baro readings in fixedwing (unless you added a module to read the baro).
Also are rather doubt that this is directly related to exposing the baro to light.
A quick guess is that since the baro is not read in the fixedwing firmware, the baro CS line is floating and it causes trouble.
Cheers, Felix