You were just right ! Access rights related to the above file were rw-rw-r.
I added write access to all (file owner and group is root), so I changed file access properties to rw-rw-rw. The camera is now connected, and giving regular pictures with EKOS either as main CCD or autoguider. Cooling is also ok. It's just so great to have this almost unknown camera back to business !
Only trouble is that if I disconnect the USB cable and connect it back again, the device number is updated, and I have to chmod 666 again the BUS/DEVICE file. There may be a way to avoid this little pain.