Comment 7 for bug 1492441

As a workaround I managed to pair successfully following the instruction in the "Pairing process" section at
https://wiki.archlinux.org/index.php/Bluetooth_keyboard
i.e.
sudo bluetoothctl -a
power on
agent KeyboardOnly
default-agent
pairable on
#enter pairing mode on the keyboard
scan on
pair MAC_OF_THE_KEABOARD_FROM_PREVIOUS_STEP
#I think now I had to enter the displayed pin on the bluetooth keyboard and press enter
trust MAC...
connect MAC...
quit
The keyboard stays connected even after the system restart.