Nivona 691 - FI Flies after pressure buildup

Hello, maybe someone here knows about the Nivona machines.

Have a Nivona 691 from the in-laws "inherited" get there new acquisition. But worked for years without problems there. Then I packed them and plugged in again with me. But now the problem. Machine starts, mill grinds, pressure is built up, shortly before reference flies the FI.
Machine tested elsewhere(without FI), device works fine again.

Does anyone know the problem or what the problem could be?

There are not many options for the machine. Every consumer to which the protective earth conductor goes. As far as I know, only the heating remains with the machine.
The best thing is to open the machine and see if it is leaking. And see if there is another consumer to which the protective earth is connected.

Thanks for the info, I'll see if I can see anything with the machine open.
LG