What is the oldest historical account of European dragons (Lindworms, Drakones etc.) being associated with Asian dragons (Lóng, Ryū)? Is the association simply because they are both serpent-like fantastic beasts or is there more to it?
Well, Marco Polo (scan down to Chapter XLIX) describes "vast serpents" that are found in the province of "Carajan" (probably Yunnan). He didn't say he actually saw them and didn't call them "dragons".
But all dragon myths probably have a common origin deep in humanity's past: A deep instinctual fear of snakes. It appears that primates are hardwired to detect snakes as one of their defense mechanisms against predators. Dragon myths were one of the ways the ancients expressed this fear, probably reinforced by spotting dinosaur bones and the like. (this last being vaguely remembered from old college lectures).