I put some quest items in the bank before going to an instance, then i withdrew the items to go turn the quest in. The NPC's consider the quest not to be complete, the question marks above their heads are greyed out and the complete quest button is greyed out.
I have the same problem with the Green Hills quest in STV: images
We seem to get it with different quests. It being only for a small number of players for different quest makes it that the devs might not pay much attention.