Once "serving" starts, I've found that if the player on the other end doesn't properly STOP playing, (escape exited or powered down) the media server will continue "serving" and get hung up - stuck and not available any more to serve again.
There seems to be no way to clear the "serving" other than to QUIT and Restart PS3MS from the Mac. (Also power down the player and restart just to make sure all the requests and serving has cleared out.)
Once quit/restarted, it's working again. Any easier way to stop and clear out a current task?