No collision detected between two gameObjects

There are several topics about non-working collision detection, but no solution worked in my case.

I would like to trace the collision between two very simple objects and only want to know at what point they collide, but it should be possible that they overlap and are not knocked back.

There is one GameObject ("Volume") that is moving and when it fades into another GameObject ("Shape") an attached sript should notify! Simple, but somehow not working. These GameObjects are created dynammically with an button event while the program is running.

The Volume is created like this:

 //---------------------------------------------------------------------
         private static GameObject createVolume( float moduleWidth, float moduleHeight, string textureName )
         {
             GameObject cube = Resources.Load( "Prefabs/Volume", typeof( GameObject ) ) as GameObject;
 
             //GameObject cube = GameObject.CreatePrimitive( PrimitiveType.Cube );
 
             // Assigns a material named "Assets/Resources/ReflectionMaterial" to the object.
             Material newMat2 = Resources.Load( textureName, typeof( Material ) ) as Material;
             cube.renderer.material = newMat2;
             //cube.renderer.castShadows = false;
             //cube.renderer.receiveShadows = false;
 
             cube.rigidbody.useGravity = false;
             cube.rigidbody.freezeRotation = true;
             //cube.rigidbody.freezePosition = false;
 
             cube.collider.isTrigger = false;
 
             // set size
             cube.transform.localScale = new Vector3( moduleWidth, 1f, moduleHeight );
 
             return cube;
         }

so, there is also the script attached to this Volume that should trace the collision:

 using System;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class GEO_VolumeScript : MonoBehaviour
 {
     //---------------------------------------------------------------------
     void OnCollisionEnter( Collision col )
     {
         Debug.Log( "COLLISION ENTER   " + col.gameObject.name );
         if( col.gameObject.transform.tag.Equals( U3D_Definitions.SHAPE_TAG ) )
         {
             U3D_ResultManager.PrintResultHit( this.gameObject, col );
 
             U3D_ModelManager.DrawHit( col.gameObject );
         }
     }
     //---------------------------------------------------------------------
     void OnCollisionStay( Collision col )
     {
         Debug.Log( "COLLISION  STAY " + col.gameObject.name );
         // count the time of collision
     }
     //---------------------------------------------------------------------
     void OnCollisionExit( Collision col )
     {
         // trigger end of collision
     }
     //---------------------------------------------------------------------
     //void Update()
     //{
     //    // this is working
     //    Debug.Log( "Update" );
     //}
 }

and the Shape is created like this:

         public static GameObject CreateShape( Vector3 position, float width, float height, float depth, Quaternion orientation )
         {
             Debug.Log( "GEO_ShapeFactory:: CreateShape()" );
 
             position.y += height;
 
             //  create shape and clone it
             GameObject shapeObject = GameObject.CreatePrimitive( PrimitiveType.Cube );
             GameObject shapeObjectClone = ( GameObject ) Instantiate( shapeObject, position, orientation );
 
             shapeObjectClone.transform.localScale = new Vector3( width, height, depth );
             shapeObjectClone.collider.tag = U3D_Definitions.SHAPE_TAG;
 
             
 
             // add material to shape
             GEO_ShapeFactory.setGeometryObjectHighlighted( true, shapeObjectClone );
 
             Destroy( shapeObject );
 
             return shapeObjectClone;
         }

So, from the script I get no response at all. All objects have box colliders added. The Volume has a rigidbody, where gravity is deselected and the position and the rotation are frozen. If I add a rigidbody to the Shape the object is knocked back in the scene, so the collision is taking place, but the script doesn't tell. I tried the Update() method there to see if there is actually anything happening - positive... so the script is there, recognized, working but without the OnCollisionEnter method... :-(