Archive Tower Giant Cell Key

[Dens Aquilae]

I dont know where it is, thus I cant rescue Logan from the cell in Duke Archives.
I read its near the exit of duke archives to the crystal cave.
Is it in the chest under the lowering staircase? Because if so, then I cant loot it for some reason. Whenever I try to loot it, the item gets placed to my feet, as if I dropped something out from my inventory. Could you help me with some instructions?

[Groen90]

Those are just prism stones, they drop on the ground because you already have the max amount in your inventory; the key is one of the chests upstairs, can't remember exactly which one, probably the one in the other room, the one that has only one chest

[Dens Aquilae]

ah, thank you, i found it