Error: The application “NoMachine.app” is not open anymore
If you're using 10.11, check this out:


  A crash occurs when executing a call to network API on OS X 10.11 El Capitan
  
  Based on the Diagnostic Report, the crash occurred during the discovering of other computers on the network and it's related to a call to network API.
  
  As a temporary solution, disabling the 'Discovering of other computers on the network' in player.cfg helped, but this is not a fix and we cannot exclude that the crash appears again. There can be other situations in which a call to the same API can be executed.
  
  Workaround:
  
  
  Edit the player.cfg file in the .nx/config directory under the user's home (~/.nx/config) on the client computer.
  Set:

&lt;option key="Discover other NoMachine servers in the network" value="false" /&gt;

  


The workaround in that page helped me. Some weeks ago they've also sent me a test library fixing the issue (I've reported the problem on their forums). You may get in touch with them or wait for an official fix.