no one has asked about this on Phabricator yet

Yes they have. See T263928: VisualEditor in 1.35 not working (404 / Permanent Loading) and T261581: Request to parsoid failed on Docker Developer Environment at 80 port which have the identical error message mentioned within them.

I have the latest version of MediaWiki

Latest version is ambiguous.

There are many threads on the support desks, such as https://www.mediawiki.org/wiki/Topic:Tjzkjkhe1voerymw / https://www.mediawiki.org/wiki/Topic:Uzwaxv6t2myhvf8o / https://www.mediawiki.org/wiki/Topic:Tlh4ayg0ycrrkzas / https://www.mediawiki.org/wiki/Topic:Toakcj2gz91ty119

You should probably add port mapping info in troubleshooting: https://www.mediawiki.org/wiki/Extension:VisualEditor#Troubleshooting

I guess it is more common now to either have a proxy or a docker container port mapping.

This is a minimum, required configuration for MW 1.35 for VE in practice:

wfLoadExtension( 'Parsoid', 'vendor/wikimedia/parsoid/extension.json' );
$wgVirtualRestConfig['modules']['parsoid'] = array(
	// local, server-server URL; must be valid within your VM/container
	'url' => "http://localhost/rest.php",
);

Contrary to what docs says you seem to need both. And I think this should be safe for almost all installations.

I think it is crucial to not that the URL is not the same as you see in your browser. It is an url for server-server communication (not user-server).

One reason the port is different is that you are running a docker image with port mapping like -p 8080:80. The other would be that you have a HTTPS proxy and so remotely you have a 443 and internally you have a port 80.

Example docker-compose.yml you can use to repeat the problem:

mediawiki:
  image: 'mediawiki:lts'
  ports:
    - '8000:80'

Nevermind. Forgot I can change that ;-)