I got it to work (Finally!!!), by doing voodoo:
I saw that the files recorded by the Hero3, while the camera was set at 60 fps, were not really at that, but at 59.94 (looking at their properties in VLC).
So, I set intensity shuttle to expect that, and tried. That didn't work. But then, I tried the often mentioned 'trick' of setting the GoPro at 120fps.
And finally I got a signal!
I don't know what's wrong with either or both devices to have to set one at 59.94 and the other at 120 fps to get them to work
I do know that I will not be touching my settings ever again in either device
So there...