There seems to be a typo in the CSS file (pro and free version):

Failed to compile.

./node_modules/mdbootstrap/css/bootstrap.min.css

Module build failed: Syntax Error 

(7:1) Unclosed comment

  5 |  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)

  6 |  */:root{--blue:#007bff;--indigo:#6610f2;--purple:#6f42c1;--pink:#e83e8c;--red:#dc3545;--orange:#fd7e14;--yellow:#ffc107;--green:#28a745;--teal:#20c997;--cyan:#17a2b8;--white:#fff;[...here is the rest of the css code...]

> 7 | /*

    | ^

  8 |

There's no problem when I go back to v4.4.4. And I can reproduce the problem with a simple node.js server (based on create-react-app) as well as in the asset pipeline of Rails 5.1.