How to debug issues with feature:
Make sure that https is working properly, by using console tools.
Https might work in browser and still be broken for some clients, check if wget https://invoice.ninja.example.org/test_headless downloads something.
One such error scenario is when you don't upload intermediate certificates to your cert chain. Browser will download them, but other clients will not.
Make sure that host is resolvable from your VM you installed invoice ninja at (and if you use docker, check that everything works from inside docker container).