I applied online. A few days later I received a mail from David, a robot, which helped me to schedule time for an interview.

The conversation with the HR manager went smoothly. We talked for about 20 minutes. I was told a little about the company and was asked to tell something about myself, my experience, technologies, that I worked with. Then I was sent a coding challenge, which I had to finish in two days.

4 days after submission I received an email with rejection and also a brief explanation as to why my solution didn't meet the requirements.

You need to write a RESTful web service.