When you get this Error: Node Sass version 5.0.0 is incompatible with 4.0.0 while trying to run your application, here is a possible solution to it.
Workaround: don't install node-sass 5.0.0 yet (major version was just bumped).
npm uninstall node-sass
Then install the latest version (before 5.0)
npm install firstname.lastname@example.org
This is the solution that currently works