I found this forum and contacted CitroJim. Jim was enormously helpful, scanning the car with the software on his laptop and after a moment, suggested that the battery may be not really charged. The computer was detecting 8.8V on the battery. It was quite embarrassing
Jim thank you for helping and for the time you spent on it, not mentioning that you had to travel to my place, as my car was stationary.
Lessons learned: a) check the battery first, use some meter; b) look through the car's manual to refresh memory;
The re-initialisation procedure of sensodrive on my C3 (e.g. after the battery was disconnected):
- switch on the ignition
- press on the brake pedal and keep pressing
- wait until you see "N" or a gear displayed (approx. 30s)
- release the brake pedal