I start autossh using a shell script. In this script there are variables for the host IP address and the port. The first time I ran the script everything worked as expected.
Later the host IP address changed. So I killed the autossh processes, changed the IP address and re-ran the script. The problem was autossh started with the old IP address!
Somehow my original script was copied to the same dir as the autossh command (/usr/bin). When I ran the new script, somehow the one in the autossh dir was run. Deleting the script in the autossh dir solved the problem.