Why would my transponder interfere with my ADF? The ADF in my plane works very well until I switch the transponder to ALT. At that moment, the ADF needle is stuck in a random position and will not show the direction to the selected NDB anymore (the transponder works flawlessly).

Where is the interference coming from? Is it because the two boxes sit on top of each other on the dashboard? Or is it due to a bad placement of their respective antennas on the fuselage? Would seem strange to me, as their respective frequencies are far apart (kHz vs. MHz).

Any norm / guidance on how to install these two systems so that they do not interfere with each other?
ADFs and transponders are often right next to each other in the dashboard, it's not the placement that's the issue. The two systems work on different sets of frequencies, so antenna placement shouldn't be an issue. If the ADF is being getting interference from the transponder in ALT mode the most likely issue is that one or the other is malfunctioning, or there is a short in the wiring between the two systems.