Hi Blair,
please open the Network pane of the Developer Tools and explore each request looking at its response: an AJAX call (xhr) would show you an HTML payload instead of a JSON one and in that HTML you'll read what your Moodle instance is complain about, preventing to output the expected JSON response.
Edit: are you talking here about https://moodle.org/mod/forum/discuss.php?d=374445 ?
HTH,
Matteo