remote debugging: failed to find free socket

I'm trying to use remote debugging but get this error in RubyMine when I try:

Error running remote-rspec: Failed to find free socket port for process dispatcher

The debug pane shows status as connected, but it immediately fails with the above error.

rdebug-ide on the remote server, which is an ubuntu vm, seems to start:

$ rdebug-ide --port 8100 --dispatcher-port 26162 -- rails s
Fast Debugger (ruby-debug-ide 0.4.17.beta16, ruby-debug-base19x 0.11.30.pre10) listens on 127.0.0.1:8100
 

It doesn't seem to run the specified script (rails s) at all though.

In RubyMine on my Mac, I'm using this "Ruby remote debug" config:

Remote host: localhost
Remote port: 8100
Remote root folder: /home/dev/repos/myapp
Local port: 26162
Local root folder: /Volumes/repos/myapp
 

The vm is configured to pass a set of localhost ports to the vm, including 8100. And using python's SimpleHTTPServer as a test, I re-verified that the vm can serve traffic to localhost:8100.

RubyMine v5.0 RM-125.65

What do I have misconfigured?