I can't understand why the result of changes() in the example file changes. I get:
[[6.90642211e-310]
 [1.01702662e-316]
 [1.58101007e-322]]
[[0.]
 [0.]
 [0.]]
with an Ubuntu 14 system that has had a lot of changes made. I've checked the same happens on pythonanywhere.com so it does not seem to just be my system is broken. I wondered if there was some strange state in cv2 I don't know about, but as commenting out the tvec=np.zeros(3) line removes the behaviour I think there is something strange here. Now I've got it down to a few lines I can find a work around  and obviously numpy and opencv are huge packages and it may be in their court, but I think it is worth someone taking a look who knows more than me.
Yours
Ian