I wasn't touching anything at that moment, except for the plastic handle of an insulated screwdriver. I was standing on an aluminum ladder, but I had rubber soles.
The wiring is rather suspect. There's a bonding bus bar in the basement where gas and water mains enter. The new gas mains isn't bonded, the new water mains is plastic. Cable TV is still bonded, as is a weird stub of old lead pipe disappearing in the ground. The bus bar is rusty and threatening to fall apart (half of it already did). From the bus bar a ground wire runs to the main fuse enclosure. At the PoCo disconnect ground and neutral are simply under the same screw, from this point all distribution is 5w. From the cutout a new cable goes to the main fuse box mentioned above. Then the risers go to every floor where a short lead to the apartment main fuses is tapped off. The riser wiring is late 1970ies PVC wires in FX conduit. Maybe the ground is disconnected somewhere, which would at least explain the RCD not tripping.
The old RCD was definitely working, it tripped on some occasions (direct hot-to-ground faults though). We carefully tried touching the ground bus bar with the back of our hands but never got shocked since. There was no voltage between Neutral and ground, but between the lifted-off dishwasher ground and the bus we measured a fluctuating voltage up to 200V.
All water pipes except for the last few m in the apartment are plastic (makes for nicely floating grounds in the apartments with older wiring
Voltage drop is the only explanation that seems possible.