Answer:
To determine the correct stresses, it is necessary to know the local coordinate system of the surface. It can be activated in the Display navigator under "Model" → "Surfaces" → "Surface Axis Systems x, y, z".
All stresses with "+" in the index represent the stress on the upper side; that is, the side of the positive local z-axis. In this specific case, this means that my represents the bending moment of the lap joint in the direction of the global y-axis and therefore, the resulting value must be 2 kNm/m.
The average in the diagram shown in the image confirms this with the value of 1.95 kNm/m (deviations can be reduced by a finer FE mesh). The average of the shear flow is 100 kN/m and thus corresponds to the applied load. Therefore, the stress σy,+ reflects the compressive stress of the lap joint on the upper side of the surface; τxy,x corresponds to the shear stress in the interface on the same side.
