Ensure that the "G0" (Position) and "G1" (Tangency) constraints are active on shared edges.
Surface cracking typically happens during the transition from a polygon mesh (STL) to a NURBS surface. It is rarely a software bug and usually a result of specific data or settings issues. quicksurface crack
: Attempt to join the surfaces into a single Solid Body. If QUICKSURFACE (or your destination CAD) cannot "knit" the surfaces, there is a physical gap that needs closing. Ensure that the "G0" (Position) and "G1" (Tangency)