Use NodeRED in an iframe behind a proxy

My NodeRed instance runs on a pc (currently ubuntu 20.04) behind a nginx reverse proxy. I installed it today with the install script, so it should be the latest stable version (i guess). Nginx config, proxy settings taken from https://discourse.nodered.org/t/node-red-server-with-nginx-reverse-proxy-howto-guide/27397:

server {
	listen 80 default_server;
	listen [::]:80 default_server;
	server_name _;

	location / {
		proxy_pass http://127.0.0.1:1880;
		proxy_http_version  1.1;
		proxy_cache_bypass  $http_upgrade;
		proxy_set_header Upgrade $http_upgrade;   
		proxy_set_header Connection "upgrade";
		proxy_set_header Host $host;
		proxy_set_header X-Real-IP $remote_addr;
		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
		proxy_set_header X-Forwarded-Proto $scheme;
		proxy_set_header X-Forwarded-Host $host;
		proxy_set_header X-Forwarded-Port $server_port;
	}
}

This works great so far with direct access. Though, I want to use another pc as proxy server for this one (ubuntu 20.04 as well), and I use nginx with almost identical settings:

server {
	listen 80;
	listen [::]:80;

	server_name s1.hiveserver;

	location / {
		proxy_pass       http://10.0.0.2;
		proxy_http_version  1.1;
		proxy_cache_bypass  $http_upgrade;
		proxy_set_header Upgrade $http_upgrade;
		proxy_set_header Connection "upgrade";
		proxy_set_header Host $host;
		proxy_set_header X-Real-IP $remote_addr;
		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
		proxy_set_header X-Forwarded-Proto $scheme;
		proxy_set_header X-Forwarded-Host $host;
		proxy_set_header X-Forwarded-Port $server_port;
	}
}

This works great as well in conjuction with my local hosts file:

<snip>
192.168.57.10	hiveserver
192.168.57.10	s1.hiveserver

with 192.168.57.10 being the IP of the proxy.
As long as i access s1.hiveserver in the browser directly, it works perfectly fine. But if I serve a simple website from the proxy itself (http://hiveserver) containing an iframe with the s1.hiveserver site, the nodered header loads, but the rest of the windows stays gray.
This is the site the proxy is serving:

<!DOCTYPE html>
<html>
	<head>
		<title>Server frontend page</title>
	</head>
	<body>
		<iframe src="http://s1.hiveserver" width="1500px" height="800px" style="margin: 50px" />
	</body>
</html>

And the result:


When loading the page, the js scripts get loaded with status 200, but the JS console shows an error:

jQuery.Deferred exception: Cannot read property 'getItem' of null TypeError: Cannot read property 'getItem' of null
    at Object.init (http://s1.hiveserver/red/red.min.js:16:15443)
    at Object.init (http://s1.hiveserver/red/red.min.js:16:14844)
    at HTMLDocument.<anonymous> (http://s1.hiveserver/red/main.min.js:16:161)
    at e (http://s1.hiveserver/vendor/vendor.js:2:30005)
    at t (http://s1.hiveserver/vendor/vendor.js:2:30307) undefined
vendor.js:2 Uncaught TypeError: Cannot read property 'getItem' of null
    at Object.init (red.min.js:16)
    at Object.init (red.min.js:16)
    at HTMLDocument.<anonymous> (main.min.js:16)
    at e (vendor.js:2)
    at t (vendor.js:2)

The error is 100% reproducable, even after reboot.
Unfortunately the error is so generic and seems very common with some Android web widget, I could not get any help from googling the error.
Has anybody an idea how to approach this? Please ask if something I wrote is unclear.

[EDIT]: Surprisingly, the error does not occur in the firefox browser, but only in the brave browser (chromium engine).