Rik's observations with his multimeter in ohmmeter mode made me curious to check my own StackShot controller with my Fluke 80 V meter. I do get similar weirdness, though my absolute numbers are different. (I didn't record them, before anyone asks, and they probably don't matter--I did not exercise great care, and there is considerable interference in the area where my StackShot is located.)
However, I was thinking that Niels could learn what he needs to know very simply by using a multimeter's continuity-testing mode. Since the StackShot fires the camera shutter by briefly closing a switch, a continuity tester placed on the StackShot's camera output should be normally open, but closed briefly when the shutter button on the StackShot controller is pressed. The way I have my controller and multitester set up, that means I get a 1/5 second beep from the tester every time I hit the shutter button on the controller. For this, it doesn't matter whether the red goes to tip and black to shell, or vice versa.
If the StackShot closes the switch appropriately, it's time to test the cable--without the camera attached. Put the shutter cable on the Stackshot controller, place the continuity tester's leads in the appropriate spots on the far end of the cable, and press the shutter button on the controller. Is there momentary continuity?
As to what the "appropriate spots" are on a Canon 6D shutter cable: If the pinouts are correct
as shown here, third picture down, it would be the pins marked "Release shutter" and "Ground." (You Canon users have it easy! Many Nikon cameras have a 10-pin release socket.)
If the StackShot and cable perform properly in this test, it's time to check the camera. If you momentarily connect the camera's "Release shutter" pin with it's "Ground pin," does the camera fire? If not, that's where the problem is.
--Chris