Hi Armin, 

Yes, I'm using *BOUNDARY_PRESCRIBED_MOTION_SET as well as applying fixed-clamped conditions to the other end. Also, at the end of applying the prescribed motion, Boundary_spc is also applied to constrain the DOFs other than the direction under displacement control. I tried applying other magnitudes of the cyclic displacements, but the problem only occurs when certain prescribed displacement curves are used (i.e., this issue does not always occur, which is weird).