Update - I resolved this at my end following the nginx config changes from this thread: [Solved] Random 502 errors when using nginx
Apologies for missing that earlier - I had tried but am not well-up on nginx so must have missed something first time I tried!
Don’t quite understand why it is an issue - I have other ajax-heavy apps on this server - but IP now running nicely, thank you 
But as above, FYI I still see this on your demo server?