Why does this keep showing up when I'm connected to a hotspot?

I currently get this message when I connect to a hotspot for long enough:

Some sort of... hotspot login? But it's saying '404 not found'.

Can somebody please tell me what this is?

Thanks.

If you are using your phone as a hot spot it may be timing out. My phone has a certain amount of time for the hotspot to stay on. Are you sure your hotspot is still on when you get that message?

Yes. Note the Wifi icon in the top right. It wouldn't appear if I wasn't connected.

The Hotspot login is not loading, looks like it could be an error on the hotspot side try forgetting the network and trying again, also i have had issue using chromium based browsers on some hotspots try firefox to login if you are not.

This is an nginx error from whomever is hosting the hotspot - https://nmcheck.gnome.org/

What steps are you taking to reproduce this message? It's not an error, it is legit saying that no web page exits on that URL. You can reproduce this on any webpage that doesn't match the URL you are trying to hit. E.g. https://www.bbc.co.uk/flabadabababayababababaaba

I get that from works guest wifi when using chromium or brave, but not when using firefox seems to be a bug in nginx, im one of the only linux users so wasn't worth networking's time to investigate and fix, since i have a workaround, but when i use chromium to url redirects to nginx? or something .gnome.org and not the internal work url, and since the device is not auth'd it can't redirect externally for the gnome shell auth. But seems there's a native firefox auth method.

wonder if the nmcheck.gnome.org is an example url placed by the vendor and its supposed to be changed when deployed.

Like I can still use my hotspot as normal even after this shows and I close it.