Ofer & Nick,
Remember, the minus Masse Light means there is a connection between 24-volts negative to the Amel bonding system. The most common issue is a Jabsco Macerator pump that has developed a leak at the seal, causing a connection to the Amel Bonding. Start by disconnecting the green/yellow wire from each macerator pump. If the light no longer comes ON, you have found it. Replace that macerator pump and reconnect the green/yellow wires.
If it was not the macerator pump and the light is ON for the (-) side, it is more difficult because the individual 24-volt breakers only turn OFF the positive side (+). For a negative side (-) Masse fault, you will need to disconnect the negative wires from the negative main battery switch. On most Amels, this is the lower switch. When the light goes out, you have narrowed your search to one main circuit.
If the (+) side is ON, try turning OFF each breaker throughout the boat one at a time. Don't forget breakers
for the windlass, winches, and breakers in the engine room. Do this with all breakers.