If you are having trouble connecting to your minecraft server it may be due to the authentication server on minecraft.net either being down or overloaded. Various errors you may get are: "Response Code 503" or "End Of Stream" or "Took too long to login" or "Failed to verify Username", "java.io.IOException: Server returned HTTP response code: 503 for URL:", "Failed to Login". The only thing you can do to avoid this problem temporarily is to set online-mode=true to online-mode=false in your server.properties file. However as this leaves your server vulnerable to possible attacks this is not recommended as a long term solution.