Okay, so after i've finished the Unlikely Valentine quest Nick goes back to Diamond city, but when I get there, Nick won't talk to me anymore, I can't activate the quest "Getting a Clue". Any hints on how to fix this?

you could try talking to his secretary or if hes sitting at the desk sit in the chair facing him

Do you have piper with you?

His secretary only says "Thanks for Getting Nick back", and he's not sitting at the desk.

No, I have Dogmeat with me.

Bring piper with you. I think she says something, but i cant remember for sure

They still don't say anything, the problem is that I don't ever get the quest marker.