[SEVERE] java.io.IOException: Not in GZIP format - constant fatal crash. can't start server at all

This error usually indicates level corruption. There's a 1/3 chance that the main world is corrupted, and if this is the case, then you'll have to use a previous backup to restore it. If not, it can be fixed by deleting the world_nether and/or world_the_end folders. If none of these fixes work for you, then you'll have to reset your server by deleting the world folder. To prevent this from happening in the future, make sure to always stop your server with the stop command, never the X. It may become corrupted again if you do not allow it to save chunks. Hope this helps!
-AngryNerd

 

well you could download the myworlds plugin and mess around there. it can repair maps for you in your server command prompt window or pail window. example commands------>world unload world_nether. then you type world repair world_nether. then world load world_nether. and for your main world. just go into your server properties file and make a different world your main one, repair it, then switch back. hope this helps for future reference!