if like me, you got to follow some ghost to his passing point to recover some book pages, you might get confused with the bridge showcased on the actual map.

The correct bridge is the one outside the forbidden forest, not the one inside the forest itself.
(Which, when casting LUMOS on crossing it, also shows some fast purple lights moving deep in the forest to the battle arena statue where there is a couple poachers roaming around)

So first stick to the path going from Hogwarts to Hogsmeade and cast LUMOS before crossing the bridge nearby the Forbidden Forest entrance:



After reaching the end of the bridge, three floating candles should appear and as you move closer, they will move away towards the Forbidden Forest, follow them:



If you avoid the encounter with the close by Dugbog on the river by the left you should arrive with no issues to the dinner setup on a small hill and the chest should appear there, open it to finish the quest:

Hope this helps!
Enjoy the game!