A month after my first installation I gained additional insight about my electrical layout.

It turns out that my apartment and my basement/underground room are not directly wired: I can turn off the meter of the apartment while I still have power in the garage and underground room.
This means that most likely only ground and neutral are shared.
In the best case the phase is hared too but has multiple elements in the middle: at least two meters.

Given the circumstances the 60Mbit/s are not too bad.
Changing the plug type in the basement fitting a new round “German” type and removing the adapter I was using earlier did not change the signal quality.

On the bad side: I tried the firmware upgrade of the adapters and now the powerline utilities can’t talk with the adapters anymore.
Yet the adapters talk to each other: I’m going to keep this setup until the endo of warranty approaches or I get a proper cat 5e cable to connect the two places.