Far from a docker export, but I think your second one is saying when a connection comes to 1880 on the host send to 1881 on the container. Assuming you have NR on default 1880 then you are trying to connect to a port without anything listening.
Pretending you are connecting to xyz:1880/ and it works, then you could change your mapping to be 1881:1880 and then xyz:1881/ should work as you would connect to the host port 1881 which would then look to the container as if you were connecting to port 1880.