Hi Nick, please unscrew a solenoid from a valve and connect it directly to the OpenSprinkler COM and Zone 1, remove the other cables beforehand. Activate the zone and you should hear a click and hum. The display should show approx. 200-300mA. If everything is ok, test the other valves in the same way. If everything works, then there must be a cable problem, possibly a short circuit.
If the solenoid does not work, then either the solenoid is broken or the OpenSprinkler is broken.
Check the french version of the handbook:
https://opensprinklershop.de/wp-content/uploads/2020/05/os-manual_2.1.9-FR.pdf
and look at page 5 “Schémas de connexion des fils de zone”
Best regards,
Stefan