Page not found after running renderd successfully

I've been trying to set up a tileserver for a few weeks now using this instruction. Yes, I'm using 18.04 if that will be a question. Had some other issues but now everything seems fine. Or doesn't it?
I managed to run renderd without errors. But now when I open my browser and try to get on the page where I should see my tiles it says "404 Not Found".
Here you can see my Logs which belong to renderd.
Command I ran: renderd -f -c /usr/local/etc/renderd.conf

That's that my debug says:
renderd[2107]: DEBUG: Got incoming connection, fd 23, number 9 renderd[2107]: DEBUG: Got incoming request with protocol version 2 renderd[2107]: DEBUG: Got command RenderPrio fd(23) xml(default), z(0), x(0), y(0), mime(image/png), options() renderd[2107]: Dispatching request to slave thread on fd 7 renderd[2107]: DEBUG: Sending render cmd(1 default 0/0/0) with protocol version 3 to fd 7 renderd[2107]: DEBUG: Got incoming request with protocol version 3 renderd[2107]: DEBUG: Got command Render fd(8) xml(default), z(0), x(0), y(0), mime(image/png), options() renderd[2107]: DEBUG: Failed to read cmd on fd 23 renderd[2107]: DEBUG: Connection 8, fd 23 closed, now 8 left

Has anyone got an idea what could be the reason why it won't work?
Also, this answer didn't help me :/