EDIT: The place I was exploring is where you fall afterwards, problem solved! 
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Hi there, before knocking 4 times on Takemura's Safe House door 303, I noticed hackable terminals two levels below.
So I went back to the ground floor to a very dark corridor. I found that one can go through the window/door by stepping onto a crate blocking the passage. However, after having looted all the objects there, I cannot go back through the same door/window again. Crouching as for going in does not help.
So I abandoned this part, restored a saved game and continued.
_____
Hi there, before knocking 4 times on Takemura's Safe House door 303, I noticed hackable terminals two levels below.
So I went back to the ground floor to a very dark corridor. I found that one can go through the window/door by stepping onto a crate blocking the passage. However, after having looted all the objects there, I cannot go back through the same door/window again. Crouching as for going in does not help.
So I abandoned this part, restored a saved game and continued.
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