XF 1.1 Response not JSON! Error

[DBA]

A user is occasionally getting the following error, on his iPhone, while viewing a thread, any ideas? Nothing shows in the error logs. Forum is on XF 1.1.5

 

[Jake Bunce]

while viewing a thread

Any thread? Can you post an example or provide specific instructions to reproduce the error?

It would be important to identify the page that he is on and what resource is being requested via javascript.

edit - and of course standard troubleshooting can be useful here (disable addons, try default style). I just want to get more information.