Reply To: Post selection interface showing the thumbnail path instead of a page quote.

Without being in the back end rummaging around its gonna be hard to say why one works and another doesnt but lets take a systematic approach – the dates on these posts are quite ancient as it seems might be the theme – what are you using for a theme? Are there any wordpress or theme or plugin updates needing to be done? Some older ones work and so do some now ones so no obvious clues there – and that json function is that a theme call or a plugin call – what are you using to discplay the posts?

First thing I would personally do is to create a new post and put the basics of your content into it and see if it borks!
1. TITLE – Test post, IMAGE upload a new copy, DESCRIPTION – short and sweet test description – NO PUNCTUATION and NO LINKS
2. If it works, swap it out with what you want it to say one piece at a time till it breaks or work
3. If that doesnt work – duplicate a working post and add your borked content to the working copy

Not much help but troubleshooting can be frustrating! Good luck!