Hi,
I found the error in DB table ach_postmeta
So if I understand it correctly
the main product have 2 images 7000 is the main product image 7902 is th attribut image was silently added to the main product
a:1:{s:14:"7000";s:4:"7902";}

So I don’t know what happened but when I added the attribute image it seems to add the attribute image to the main product too but only in DB as there was just one main product image for this product in the backend.

And because the image was one of the attribute this image was preselected instead of the main image.

I don’t know how your ajax determine the main image to display …
Anyhow, I have manually edit the DB entry to fix it.