Wie ist diese Wohnung ohne Adresse legal und wo kann man sie melden?

The commune of Lorentzweiler might be interested. Either they don’t want you to register because someone’s already registered there (and wants to stay registered to avoid e.g. tax authorities to get wind of them having moved) or they simply Fed up a renovation.

The latter seems more likely. On all the pictures, none of the rooms are full height. For „surface habitale“ rooms that are „mansarde“ get only account for 50% of their surface. That place probably has 30 sqm but due to that rule (which makes perfect sense), it ends up being a 15 sqm place. It probably doesn’t meant the legal requirements for housing.