A PostCSS Plugin was passed as a function using require(), but it must be provided as a string

[averri]

Starting the server with npm run dev fails with the following message:

alex@alex-ubuntu:~/dev/src/material-kit-react$ npm run dev

> [email protected] dev
> next

ready - started server on 0.0.0.0:3000, url: http://localhost:3000
Error: A PostCSS Plugin was passed as a function using require(), but it must be provided as a string.
Read more: https://nextjs.org/docs/messages/postcss-shape
Error: Malformed PostCSS Configuration
    at /home/alex/dev/src/material-kit-react/node_modules/next/dist/build/webpack/config/blocks/css/plugins.js:156:19
    at Array.forEach (<anonymous>)
    at Object.getPostCssPlugins (/home/alex/dev/src/material-kit-react/node_modules/next/dist/build/webpack/config/blocks/css/plugins.js:130:13)
    at async Object.__overrideCssConfiguration (/home/alex/dev/src/material-kit-react/node_modules/next/dist/build/webpack/config/blocks/css/overrideCssConfiguration.js:15:28)
    at async Object.getBaseWebpackConfig [as default] (/home/alex/dev/src/material-kit-react/node_modules/next/dist/build/webpack-config.js:1525:9)
    at async Promise.all (index 0)
    at async Span.traceAsyncFn (/home/alex/dev/src/material-kit-react/node_modules/next/dist/trace/trace.js:74:20)
    at async Span.traceAsyncFn (/home/alex/dev/src/material-kit-react/node_modules/next/dist/trace/trace.js:74:20)
    at async HotReloader.start (/home/alex/dev/src/material-kit-react/node_modules/next/dist/server/dev/hot-reloader.js:325:25)
    at async DevServer.prepare (/home/alex/dev/src/material-kit-react/node_modules/next/dist/server/dev/next-dev-server.js:289:9)

Environment:

• Nodejs: v14.15.4
• Ubuntu 18.04