Specifically it writes to /app/frontend_build/env.js
. Alternatively if you can't get it working you can manually modify the file:
window.env = {
API_URL: '/certwarden/api',
SHOW_DEBUG_INFO: false
};
I realized the error is being ignored by CW when this fails so I've opened issues to fix this so the specific error will be logged in the next version when a problem occurs.