Webpack has been initialised using a configuration object that does not match the API schema.

On Windows, when running yarn (custom action "start") to start the dev server, on the step that starts the webpack-dev-server, I had the following error message:

$ webpack-dev-server --debug --hot --progress --colors
 10% building modules 2/2 modules 0 active

 Invalid configuration object. 

 Webpack has been initialised using a configuration object that does not match the API schema.

 ... 

This was not an easy one to understand, but it turned out to be very simple.

When I had CD-ed into the React app folder, I had typed the foldername with only lower case characters. The foldername was originally typed with a capital first letter. Webpack did not like that.

Solution:
cd ..
cd Folder
yarn start