Zonkes

The issue remains the same, theme change and plugin disable was the first thing I tried, since thats basically the go-to solution to see whats causing problems usually.

Bit confused.

I added the code to the plugin file.

Is there not some query I can run to re-attach all the old thumbnails back to their posts.

Otherwise supposedly I have to :

Delete all my old thumbnails from /uploads
Run SQL query to delete them as featured images
Run Auto Post Thumbnail again to generate thumbnails.

I remember that when I first ran autothumbnail it attached the full size images into the posts and not thumbnails. So I ended up having to manually add everything.