I'm just a simple regular user of vue-cli. I updated my project today to 4.0.1 but I get a yellow warning on npm run serve saying "A new version of sass-loader is available. Please upgrade for best experience." So I upgrade firstname.lastname@example.org->8.0.0 but then I get crashes same as @pdoria above.
I added that css stanza to my vue-config.js but it does not fix the problem.
This bug should be re-opened and the title changed to Vue-cli 4.