I finished the task of sneaking to the sarcophagus... now I must report back to the prince (as stated in the quest log) I've read and saw on youtube that chunk the security guard has to say something before I go see Lacroix... He just stands there and I can't talk to him nor use the elevators (no I'm not nosferatu and I can't get there from the sewers anyway I tried...)

Please open the console with the ~ key from the main menu, type G.Story_State and post the result here.

From the main menu as asked:
] G.Story_State
Traceback (most recent call last):
File "<string>", line 1, in ?
NameError: name 'G' is not defined

It should be done after loading a save game cos it's related to a save game state.
Also in your case it should be set to 15 (if it's not then you can set it with this: G.Story_State=15).

I also did it while in a game from the menu (main menu requires to quit the saved game, that is why I did it from the main menu because I was asked from the ''main menu'') It was indeed 15. But that does not mean/solve anything.

Well it "solves" the answer to Wesp's (who wants to help you, but needed to know) question.

of course! and I'm grateful for all your help! I was just trying to say ''back to square 1'' ok I'll try that when I get home, I work 2 double shift and I have to sleep... at work. and when you say ''from outside'' you mean the building? santa monica? where should I be to be sure to not fucked up my game? (anywhere? I just exit the building/city and come back?)

Yes, you need to reenter the map otherwise it won't work.

that didn't do anything
LOL I just fixed it by turning off his monitor and he said don't touch that! and now I can talk to him XD

Genius!

Still, I wonder what went wrong.

Can someone make a comment about: turning off his monitor fixes the problem so I can mark it as ''solution''?

They're behind his desk. I know I changed views on one of them already; as I recall, it works just like the one on the Dane.

Alcaray's suggestion did NOT solve it for me and I have found my own solution/a solution that actually works nowhere else so I'm just posting it around here and there. If you can't speak to Chunk to progress the story AT ANY POINT then go to the back behind the elevators. On the right side, there will be a door with a Lockpick 10 skill check. Open the console with ~ and type in noclip. Phase through the door and open the console and type noclip again to disable it. There is an elevator there that you can personally call up or down without needing to talk to Chunk.