This generally means the mission is not loading correctly. Go the the directory where your server.cfg file is and open the console log. If you have the line in your server.cfg 'logFile =' then the console log will be named whatever is there, otherwise the console log will be named with an underscore and 4 digits (_1234). If you find nothing in the console log then open the server log (.RPT file also in the same folder as the server.cfg) and look for an error.

I would only consider Flight2Minimums solution as a last resort. This can normally be solved by fixing the issue causing the mission not to load.