np

It is friendly to say to a person who has interrupted you, or cut you off, or woken you up, or missed an appointment, that the problem they caused you is no problem. By minimizing the wrong done — by saying that it was no problem — you both acknowledge the apology and express forgiveness. Perfect. But, "no problem" is absolutely the wrong way to reply to an expression of thanks — for the simple reason that saying "thank you" isn't, or shouldn't be, a veiled way of making an apology.