if a refrigerator requires more electricity when it is installed, this strongly indicates that it cannot get rid of the waste heat! If the waste heat cannot escape, moisture cannot escape either --> mold
Are there any ready-made solutions to install active fans in the socket air intake? (regardless of other measures against mold)
I don't know if there is exactly "the" solution for that. But I would look in the area of a PC case fan, cost around 10-20€. They need 12V, plus a suitable power supply (with a similar keyword in an electronics shop of your choice). On Amazon you will find something with "PC case fan with power supply". Either really with a power supply or also a USB connection, which you can plug into any socket with the appropriate small power adapter.
I would exclude device failure; the fridge/freezer unit is now positioned (without a built-in cabinet) in the middle of the kitchen and is operational. There is no water leaking anywhere. The wall behind / beside the fridge was full of water droplets, as if those spots had been sprayed with a spray bottle. By the way, also inside, that is between the device and the built-in cabinet, there were these droplets. This is no longer the case now, neither on the device nor on the wall. That’s logical. The fridge is now ventilated all around.
From the way you put it, it’s not a device error but the wrong device. You probably have a fridge that does not have a heat exchanger on the back but dissipates heat on the sides and the rear. These devices need air space on the left, right, and back. Just a guess.