port forwarding fails in background

[Kristoph]

If I setup port forwarding like this ...

ssh -L 8081:localhost:8080 ro...@12.34.56.78

It work fine.

---

If I setup port forwarding like this ...

ssh -N -f -L 8081:localhost:8080 ro...@12.34.56.78

I get (with -v -v) ...

[omitted for brevity the startup sequence which looks fine, the key is
accepted, etc.]
debug1: Connection to port 8081 forwarding to localhost port 8080
requested.
debug2: fd 5 setting TCP_NODELAY
debug2: fd 5 setting O_NONBLOCK
debug1: channel 1: new [direct-tcpip]
channel 1: open failed: connect failed: Connection refused
debug1: channel 1: free: direct-tcpip: listening port 8081 for
localhost port 8080, connect from 127.0.0.1 port 56343, nchannels 2

What can cause this?

Thanks ]{