I took another look at this and it looks like you're right the actual image is there, but the preview/optimized version is broken. It isn't showing access denied or forbidden for the preview, it is actually responding with an 'OK' 0 byte response:
curl -v 'https://ce-forum.s3.amazonaws.com/optimized/1X/51fb2a7eb1deb7b2993cf5c5b9023b056cf177ce_1_666x500.jpg' -H 'Pragma: no-cache' -H 'DNT: 1' -H 'Accept-Encoding: gzip, deflate, br' -H 'Accept-Language: en-GB,en-US;q=0.8,en;q=0.6' 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8' -H 'Cache-Control: no-cache' -H 'Connection: keep-alive' --compressed
< HTTP/1.1 200 OK
< x-amz-id-2: ZSxjbcikmg5E1658i5Wt+Nu+mp00ngEYwnhP+i6MMuuU8R/Bsp8bx0FOCqGJn5je9ZTFvTTkeno=
< x-amz-request-id: 8BE817FFED1495DF
< Date: Thu, 06 Jul 2017 15:03:39 GMT
< Last-Modified: Tue, 04 Jul 2017 21:04:13 GMT
< ETag: "d41d8cd98f00b204e9800998ecf8427e"
< Accept-Ranges: bytes
< Content-Type: image/jpeg
< Content-Length: 0
< Server: AmazonS3
It would be interesting to see if there is a file in the S3 bucket that is 0 bytes, or if it is missing but the server is responding with OK even though there is no data.