Thanks 
In config/gunicorn.conf I set daemon = False (its very case sensitive so setting it to false produced an error 
).
Now when running seahub it told me it needs some modules:
ImportError: No module named certifi
Error:Seahub failed to start.
Please try to run "./seahub.sh start" again
So I installed the two missing modules
pip install certifi idna
and it’s working now 
Ok, I still had to change conf/gunicorn.conf again and set daemon = True again, but now everything is as it’s supposed to be 