No problem means: Do not worry about me suing you for giving me a service, although you and I know that if anything went wrong as a conseqence of this free service, I would be entitled to sue you. Since we do not have a written agreement about that service you just gave me, I say no problem as a way of assuring you that those are not my intentions.
So, I think that `No problem` was introduced when lawyers took over this country
Aurel