Symfony Webpack: Could Not Find the Entrypoints File from Webpack

A relatively fresh Symfony 4.1.7 project after installing Webpack Encore throws the error

An exception has been thrown during the rendering of a template ("Could not find the entrypoints file from Webpack: the file ".../public/build/entrypoints.json" does not exist.")

where the template includes {{ encore_entry_link_tags('app') }} when launching

What have I missed?

$ yarn encore dev
Running webpack ...

 DONE  Compiled successfully in 1974ms

 I  3 files written to public\build
Done in 3.33s.

.../public/build contains

app.css
app.js
manifest.json

local Symfony versions:

symfony/webpack-encore-bundle       v1.0.0
symfony/webpack-encore-pack         v1.0.3

webpack.config.js:

var Encore = require('@symfony/webpack-encore');

Encore
    // the project directory where compiled assets will be stored
    .setOutputPath('public/build/')
    // the public path used by the web server to access the previous directory
    .setPublicPath('/build')
    .cleanupOutputBeforeBuild()
    .enableSourceMaps(!Encore.isProduction())
    // uncomment to create hashed filenames (e.g. app.abc123.css)
    // .enableVersioning(Encore.isProduction())

    // uncomment to define the assets of the project
    .addEntry('app', './assets/js/app.js')
//     .addEntry('js/app', './assets/js/app.js')
//     .addStyleEntry('css/app')
//     .addStyleEntry('css/app', './assets/css/app.scss')

    // uncomment if you use Sass/SCSS files
    // .enableSassLoader()

    // uncomment for legacy applications that require $/jQuery as a global variable
     .autoProvidejQuery()
;

module.exports = Encore.getWebpackConfig();