Turned out I had a running process that needed to be killed.. on the Linux host (as root):

ps axw | grep adb

look for a process running with adb and "fork-server" in its name. Locate the process id ("PID") at the beginning of the row. Then kill it with

kill <pid>

Next time you start adb, you should see "starting server" before the action you requested. Then it should work.