In case someone has the same problem as me, here is a solution.

In xo-web/node_modules/bootstrap/package.json line 46
change from "_spec": "bootstrap@^4.0.0-alpha.5",
to "_spec": "[email protected]",

In /xo-web/node_modules/bootstrap/scss/_nav.scss
delete line 7: "display: flex;"

then do npm install and npm run build