Thanks to Donald (from the Now Software support), here is the fix :
disabling the Now Server Manager from the Login Items of the user account did the trick.
Here follows his explanation :
Make sure Now Server Manager is not loading from Login Items or Startup Items. Unlike the version 4 Public Event Server and Public Contact Server applications, it is un-necessary to launch Now Server Manager in this way for the servers to run. In fact, it will tend to cause problems for the servers. So, if Now Server Manager is set to launch in either of these ways, remove that setting and restart the computer. Then open Now Server Manager and unlock it to check the status of the server.
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