Weird JSON page loading

[DanologyUK]

I use Firefox as my daily browser. If i leave the website for idle too long, the page changes to the screen below.

Is this some kind of issue with the forum as the URL does change in my browser because if I refresh, this page stays loaded.

https://forum.netduma.com/?app=core&module=system&controller=ajax&do=instantNotifications&csrfKey=7738584cb0f56afa861cdb32ebc7aee3&notifications=0&messages=0

[Netduma Fraser]

I've literally been getting the same thing, I've already passed it over to a colleague to have a look