Some update after further testing :
I tried to put SQM OFF in config.h : no success : frequent reboots whatever the status of the 2 power relays (On or Off). No visible sign of these reboots when the relays are OFF (they remain OFF) except that the Up Time stays at 00000 min.
I tried to put all the I2C bus sensors OFF in config.h :
What remains a bit strange to me yet is the fact that when I returned to my initial set up with no weather function activated (only ROR panel and Power relays panel), the system was stable even with the I2C SDA and SCL wires plugged in...
Is there a mix of software and hardware causes ?
I will let you aware of the future outcomes.