npm stuck at "Starting the development server..."

I know this topic has been created before, but nothing I tried could fix it. The problem is precisely the following. I have a react script on an AWS ec2 server, that I want to execute automatically, whenever the instance is starting. For this purpose, the following script is executed at the start of the AWS server:

#!/usr/bin/python3
import time
import shlex, subprocess

args = shlex.split('sudo su ubuntu -c "/usr/bin/npm start --prefix /home/ubuntu/my-app > /home/ubuntu/output.txt 2>&1"')
subprocess.Popen(args)

When I run the script manually, everything works just fine. But whenever it is run during the server start, I get the following log:

> my-app@0.1.0 start /home/ubuntu/my-app
> react-scripts start

^[[34mℹ^[[39m ^[[90m｢wds｣^[[39m: Project is running at http://172.31.14.57/
^[[34mℹ^[[39m ^[[90m｢wds｣^[[39m: webpack output is served from
^[[34mℹ^[[39m ^[[90m｢wds｣^[[39m: Content not from webpack is served from /home/ubuntu/my-app/public
^[[34mℹ^[[39m ^[[90m｢wds｣^[[39m: 404s will fallback to /
Starting the development server...

That's all - nothing happens. Does anybody have an idea how to fix this? I thought it had something to do with the fact, that it is started from root. So I tried to fix that by using sudo su ubuntu -c, but it doesn't help either.

from npm stuck at "Starting the development server..."