Omg, I just have to share this…
I was having the same problem and I went literally crazy because I couldn’t find the problem. I tried everything and my thumbnails still were not showing. I was already thinking of deleting my shop.
Untill I saw this message you found a conflict with the “WooCommerce Product Archive Customiser” plugin. I use this plugin too, so I deactivated it and now all of my thumbnails are back!!
I can’t tell you how happy I am right now, so I just wanted to thank you for posting this! 🙂
