So, I tried installing this module yesterday evening, upon finding that I got the same ‘Loading…’ message as described by @kj3rra and @jonaslaeret . Today I spend several hours trying to figure out the problem. I ended up running the mirror in server mode, and found to my surprise that on my PC the Ruter information was showing without a problem.
This told me that the problem was either with Chromium or with Electron. Running

sudo apt-get update chromium-browser

didn’t seem to do the trick.

I dug into the Electron documentation and found that adding the following to the config.js file makes the Ruter information show in Electron:

electronOptions: { 
		webPreferences: {
			webSecurity: false
		},
	},

Now, I get data from both Møllefaret and Røa T-bane :D