Hard to tell -- but if json.parser.online.fr is saying there is a parsing error then that could be it. Note, make sure you are copying from the Response tab, not the JSON tab.
If there truly is a parse error then this is likely breaking the Ajax functions of the CMS. The next step is to figure out what is causing it, and where ...
EDIT: It'd quite easy to debug JSON yourself, even if you're not a developer. It's a simple 'key: value' dictionary format, and if you've ever worked with CSS then it's very similar (obviously without the selectors).
Ya I was copying from the JSON tab, not the response tab.
Doing it the proper way, it comes back clean.
Strange. Not sure what's causing this issue.
I have another install on the same server and just uploaded all these project files into it and installed it. Works fine. So it makes more sense to just start fresh and hope it doesn't pop back up. Luckily I didn't have any content really, just sitetree defined and I can easily just duplicate that again.
The ticket I started was marked as critical by Ingo as he can replicate the issue, so it's not a total lost cause.
Thanks for all your help anyway and hopefully this doesn't come back mid project. Fingers crossed.