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 localhost:3000 and 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.