What are the likely stresses that caused the uplift of the Harz mountains? Was the uplift connected to another orogeny, lithospheric-scale folding or a mantle process?
The intraplate compression responsible for this uplift has had a couple of explanations over the years; historically, it's been ascribed to Alpine collision, i.e. the collision of the Alpine orogen with the Europe. However Kley & Voight (2008) reviewed evidence and concluded it was the effect of Africa-Iberia-Europe convergence:
... the onset of intraplate contraction coincides with an important change in relative motion between the European and African plates. At ca. 90 Ma, Africa's SSE-directed sinistral transform motion relative to Europe changed to NE-directed convergence. This agrees well with the timing and kinematics of intraplate thrusting in central Europe. Structures of similar age and kinematics occurring in southern France, Spain, and North Africa suggest that the Late Cretaceous pulse of contraction was caused by pinching west-central Europe's thin lithosphere between Baltica and Africa. Only since the onset of N-directed thrusting in the Alps in Paleocene or Eocene time are the kinematics of the Alps and their European foreland compatible, indicating that mechanical coupling between Africa-Europe and the Adria microplate had been achieved.