npm run dev fails : ValidationError: Invalid options object

This is not an issue with webpack or webpack-dev-server itself, but with the copy-webpack-plugin plugin.

With the update to the 6.x major version came a breaking change: instead of just passing the array with the config patterns directly to the CopyWebpackPlugin constructor, your now have to wrap it in the patterns property of an object and pass that.

Old syntax:

    new CopyWebpackPlugin(
      [
        { from: 'src/xxx.ext', to: 'dist/xxx.ext' },
        { from: 'src/yyy.ext', to: 'dist/yyy.ext' }
      ]
    )

New syntax:

    new CopyWebpackPlugin(
      { 
        patterns: [
          { from: 'src/xxx.ext', to: 'dist/xxx.ext' },
          { from: 'src/yyy.ext', to: 'dist/yyy.ext' }
        ]
      }
    )

They did that because the constructor now supports an additional options property:
https://webpack.js.org/plugins/copy-webpack-plugin/#options-1