How can I get rid of "creases" between collision boxes on adjacent meshes?

Sure, here they are. In the first two pics you can see how the collisions are structured on two platforms that I use. Note that the boxes at the bottom have the same z positions and height, as shown in the editor. In the last pic, you can see how they are positioned in the level. In the editor, you can see that they have the same z and y coordinates.