The only place we use "Security error" is in the thumbnail_editor non-bundled addon. Is that installed?

"Fresh install via automatic install. Created a new, tried" - I think you're missing the word 'attachment' in this sentence. Please confirm.

No idea why I installed the thumbnail editor, but that was indeed the culprit. Uninstalled that addon and everything is fine.

Cheers Chris.

Okay, it was a bit of a confusing issue to parse, but I think 2 things were going on here...

1) Bugs in thumbnail_editor addon. These are now fixed in git.

2) Misunderstanding about 'Image' field. This is to set a representative image for that article only, shown adjacent to the news post in the news listings. It's not adding it to the set of images or showing on the article itself. I have amended it to be labelled 'Representative Image' instead. I also spotted and fixed a typo in the field description.
You should be using the attachments feature I'd guess (not knowing your intentions).