For 3 recent days, I was just camping that exact spot for hours long straight, but no avail. Then finally I decided to do what Henderman suggested and worked for me (I got my all ways up to 555 level, done quests mentioned above and learned 2nd cooking recipe per way). When I came back, I have noticed an exclamination mark on my mark on the destroyed bridge and could loot the sign. If you all have managed to get 600 cooking AND all ways maxed, BUT CAN'T START THE QUEST, try to run for a moment WoW without any addons on and see, if it helps.