I can confirm that this has resolved the Launcher 404 problem for me too.Left-click on the title bar (in my case my email address was visible there in deep red) and a menu pops up that has a language option near the bottom. Select english and restart.
Yeah, I suspect the launcher constructs a URL that contains the system's language setting (something like launcher.frontier.co.uk/foo/nl/bar.html). But since nl isn't a supported language in ED it gets a 404.I can confirm that this has resolved the Launcher 404 problem for me too.
I think it has to do with the operating system language not being english (mine is dutch)