Hello @stephane-clavel,
it seems you have not some kernel modules loaded. Try lsmod command - you should see unipi, at24 and i2c_dev modules loaded. If not, follow the steps below.
As the first step, try to reinstall appropriate package (assuming you have the unipi repository already sourced):
sudo su
apt-get install unipi-kernel-modules
As the second step, reboot the controller.