I know you're getting tired of this but there is 1 more way I tried that is working for me...
Code:
http://[ecximg2]+\.[images-]*amazon\.[coma]+/images/I/([\w.]+)[.jpg]
or
Code:
http://[ecximg2]+\.[images-]*amazon\.[coma]+/images/I/([\w\.]+)[.jpg]
The problem is the "period" in the image:
51A1Zvu3cVL._SL500_AA300_.jpg
On some servers (like mine) it works fine, on others it doesn't. I've seen this problem with other definitions before, specifically the short youtu.be URL.