Status of this morning : There are still reboots, even with the the relays HIGH (i.e. no 220 V AC current transiting the output terminals of the relay).
Let me make it clearer : switching the relays has never caused the system to crash. I had just the impression that the system stability could be affected by the fact that current goes through the relay output terminals or not. If I had a hardware problem with my 8xrelays card, how would my initial set-up (with WEATHER OFF) be completely stable whatever the status of the relays ?
To go slowly I switched OFF all the sensors of the I2c bus in config.h. They are still physically connected. I just kept ON the rain and winspeed sensors.
I have also reformatted the SD card (Fat32 - no quick formating) and placed a freshly downloaded Chart.min.js from the page indicated in config.h, renamed Chart.js, in the root directory.
I am going to put WEATHER CHARTS ON to see if something is displayed. Done, System sttill stable but charts are still empty :
Despite these empty charts, the system is stable since 153 minutes, with the 220 V AC current passing through the relay output terminals :
I need to get these charts correct with these very basic sensors connected before I go further, isn't it ? What would you suggest as further action ?