Amazon S3: Missing Thumbnails

S3 https://s3.amazonaws.com/bucket_name/
pThumb 2.3.3
Revo 2.3.3
NewsPublisher 2.0.2

I setup an S3 Media Source which works fine (front- and back-end) except that the thumbnail preview is missing, both in the Manager and on the front-end with TinyMCE/NewsPublisher.

As suggested in the docs http://rtfm.modx.com/revolution/2.x/administering-your-site/media-sources/media-source-types/media-source-type-s3 I'm using a trailing slash on the AWS url, but instead of a thumb it's showing some message about phpThumb.

The problem persists on both the remote (Linux) and local (MAMP PRO) servers. [ed. note: todd.b last edited this post 11 years, 7 months ago.]

Do you have any errors in the MODX Error log, or network activity that might give us a clue regarding the problem.