I think that this should be documented, I remember having personally risen the issue.
Another recent example (from the dev list :-) ):
I guess that something could be put on Server help or, better probably, on Understanding Errors.
I propose this as raw material:
Typically this means that you have a zombie process running in the background.
This can happen after experimenting a server crash or also when putting to sleep the computer while the server is running.
You can solve the problem by selecting the 'K' button on the server window or by evaluating Server.killAll, which hard kills any scsynth still active.
Then you can reboot the server.