I have the same issue as described here.
Suddenly, out of the blue, when I use
yarn start the main.css is not being found anymore, I just see the browser default rendering in
example.test. I have to use
yarn build instead, which causes another interesting error: mainly that the js router does not work anymore.
This worked fine every day for over two months now, and I have not updated anything. It just stopped working yesterday.
None of the things described here helped me with this issue.