One possible reason would be a faulty controller. For example, consider the Giemme controller pictured below:
< Pic above courtesy of farmroast, posted in Giotto Problem
Note in the circuit diagram that if the controller fails to switch the K1 relay, it will neither pump nor heat, and you get that flashing light.
This pictured controller is just one of many that might be found in a Rocket Giotto Classic, but I believe the other ones that support the flashing low reservoir light would work similarly.
When the reservoir is low, the controller is designed to not
switch that relay, so the first thing to try would be grounding the reservoir sensor wire to the controller's reference ground wire, which I think you have already tried. A defective transformer, relay, or capacitive sense circuit inside the controller might cause the failure you're seeing. Unfortunately, controllers are expensive and you don't generally want to replace them until you know they are the culprit. For folks in the US I often suggest sending them to jpboyt (Boyt Enterprises
) for diagnosis and repair. You may find someone closer to home that can do that.