TypeError: node.getIterator is not a function when using VuePress

I'm following https://v1.vuepress.vuejs.org/guide/getting-started.html#quick-start to generate a new VuePress site. I did the following (I'm using Node 15.4.0):

npx create-vuepress-site
cd docs
npm install
npm run build

I'm getting:

TypeError: node.getIterator is not a function at LazyResult.visitTick (/home/marcin/tmp/vuepress-testing/docs/node_modules/optimize-css-assets-webpack-plugin/node_modules/postcss/lib/lazy-result.js:528:33) at LazyResult.runAsync (/home/marcin/tmp/vuepress-testing/docs/node_modules/optimize-css-assets-webpack-plugin/node_modules/postcss/lib/lazy-result.js:408:30) at LazyResult.async (/home/marcin/tmp/vuepress-testing/docs/node_modules/optimize-css-assets-webpack-plugin/node_modules/postcss/lib/lazy-result.js:221:30) at LazyResult.then (/home/marcin/tmp/vuepress-testing/docs/node_modules/optimize-css-assets-webpack-plugin/node_modules/postcss/lib/lazy-result.js:206:17) at OptimizeCssAssetsWebpackPlugin.processCss (/home/marcin/tmp/vuepress-testing/docs/node_modules/optimize-css-assets-webpack-plugin/src/index.js:81:8) at Object.processor (/home/marcin/tmp/vuepress-testing/docs/node_modules/optimize-css-assets-webpack-plugin/src/index.js:20:18) at /home/marcin/tmp/vuepress-testing/docs/node_modules/last-call-webpack-plugin/src/index.js:150:10 at arrayEach (/home/marcin/tmp/vuepress-testing/docs/node_modules/lodash/_arrayEach.js:15:9) at forEach (/home/marcin/tmp/vuepress-testing/docs/node_modules/lodash/forEach.js:38:10) at OptimizeCssAssetsWebpackPlugin.process (/home/marcin/tmp/vuepress-testing/docs/node_modules/last-call-webpack-plugin/src/index.js:147:5) at /home/marcin/tmp/vuepress-testing/docs/node_modules/last-call-webpack-plugin/src/index.js:178:28 at AsyncSeriesHook.eval [as callAsync] (eval at create (/home/marcin/tmp/vuepress-testing/docs/node_modules/tapable/lib/HookCodeFactory.js:33:10), :25:17) at AsyncSeriesHook.lazyCompileHook (/home/marcin/tmp/vuepress-testing/docs/node_modules/tapable/lib/Hook.js:154:20) at /home/marcin/tmp/vuepress-testing/docs/node_modules/webpack/lib/Compilation.js:1409:36 at AsyncSeriesHook.eval [as callAsync] (eval at create (/home/marcin/tmp/vuepress-testing/docs/node_modules/tapable/lib/HookCodeFactory.js:33:10), :9:1) at AsyncSeriesHook.lazyCompileHook (/home/marcin/tmp/vuepress-testing/docs/node_modules/tapable/lib/Hook.js:154:20)

I also noticed that the same error started to appear on my existing VuePress site, which worked without issues before (I think even yesterday). What is the problem? Am I able to get around it somehow?