Just upgraded to 2.1.2 from 2.1.0. I tried to open one of my configurable products but I get this error message Cannot gather stats! Warning!stat(): stat failed for / I put the image into the location but it still won't open or see the file. The simple products associated with the configurable all pull their images fine.
I am experiencing this issue too. But I have figured one thing: Suppose that I have 4 simple products, all the 4 products are having same images. When I delete 1 image from a product, the other 3 products will display the error "Cannot gather stats! Warning!stat(): stat failed for". I've got this error in both 2.1.0 and 2.1.2. I think this should be a bug and Magento team should fix it ASAP!
Not sure if this is at all helpful but I kept getting that message and being unable to edit my products which led to me deleting two of the broken products and reimporting them, which caused my site to go down in the front and backend, my host nexcess did a restore and noticed that my permissions had gone all wonky so they reset them and I have not seen this error again today. Hopefully this was all due to a permissions error?
Magento team already knows about this issue. They have already developed a solution for that, probably it will be released in next update. But if you really need solution here and now, then you can diff your the code in Magento/Catalog/Block/Adminhtml/Product/Helper/Form/Gallery/Content.php with this, and update the function getImageJson() content.