We had created to fallbacks (https://wordpress.org/support/topic/error-in-category-post?replies=30): Your uploaded thumbnail size is smaller than your Thumbnail settings in the Media panel and your uploaded thumbnail size is bigger than your Large image setting in Media panel. But as you mentioned we fixed this two issues in previous versions.
I tested both kinds of occured bugs in previous versions: To small uploaded images and to big uploaded images. For me it works well with 4.6.1.
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Another idea to find the post(s) with the image which doesn’t work is to set the option ‘Number of posts to show:’ in the Filter area to 1, 2, 3, …
Can you write us our widget settings in the Thumbnail area and your used settings under Settings > Media (https://www.dropbox.com/s/v0gg5s5nfwr7f2t/MediaSettings.PNG?dl=0)?
Michele write us on Facebook, the problem occurre when a post had added a thumbnail and the image is deleted on the physical directory only in the databse. She had a lot of old blog posts RR transfered from my non-mobile site.
She will add featured images to those posts and this fixes it.
We will release a patch asap.
