How to fix Error:0 on a Minecraft Server?

Well, I seems like a protective plugin isn't allowing you to preform those commands… Either 1) You don't have a specified location to send someone when they do /spawn or 2) CoreProtect or WorldGuard is messing up something with essentials… I recommend you reinstall essentials, and make sure the plugins are for the correct version of bukkit you have. If none of this works, then i have no idea what is going on. Hope this helps.