Allow traffic on port / ubuntu - raspberry-pi

Question

I'm getting crazy over this problem for over a week now.
I have a Raspberry Pi (Raspbian lite) directly connected with an ethernet cable to my laptop (Ubuntu), and I want to communicate with my application (running on the pi) over a tcp socket. My Laptop has the ip-adress 10.42.0.1, the pi has the ip-adress 10.42.0.241, I want to send data from to laptop to the pi on port 3000. I'm able log into the pi over ssh (ssh pi#10.42.0.241), and I'm able to ping the ip adress (ping 10.42.0.241 has a positive result).
To test if the port is open and I can send data from my latop to the pi I did on my laptop
$ nc -vl 10.42.0.241 3000
But I got the error
nc: Cannot assing requested adress
I understood that I have to open the port on my laptop and on the pi, so I did on my laptop
$ sudo iptables -A OUTPUT -p tcp --dport 3000 -j ACCEPT
and on the pi
$ sudo iptables -A INPUT -p tcp --dport 3000 -j ACCEPT
and I got an entry in my iptable (on my laptop in the CHAIN output, and on the pi in the CHAIN input) with the following entries
target = ACCEPT; prot = tcp; opt = --; source = anywhere; destination = anywhere; tcp dpt:3000
but the error message stays the same. I can see that there is nothing else blocking the port, because i can do on my laptop (remember, laptop-ip is 10.42.0.1)
$ nc -vl 10.42.0.1 3000
and then in another terminal
$ nc -v 10.42.0.1 3000
which is basically a localhost-connection - this is working perfect.
How do I get this working the pi? Any help is so much appreciated!
I also played around with ufw and some ip-routes, but nothing has helped.
Thanks a lot!
Mario