There are many forum discussions about Http Errors. This blog is collective information about most common HTTP errors occurring during XI implementations and possible tips to solve the problems.
Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.
Because of J2EE application com.sap.aii.af.ms.app not active
• Try to (re)start the application using the Visual Administrator
Choose Server –> Services –> Deploy –> View on Application
or restart the J2EE engine
• The problem is that not all J2EE services can be started by the J2EE. i.e Start the Visual Administrator and select Server->Services->Deploy in the tree on the left. On the right-hand side, choose the Runtime tab page. You see a tree in the right window with all applications if you select the APPLICATION radio button. Check if they are running, otherwise choose Start. Usually the J2EE engine starts all services automatically.
• Refer SAP Note 803145,807000,791655
Reference: For Http code description–www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
http://service.sap.com/instguidesnw04 -> Operations -> SAP XI -> Troubleshooting Guide – SAP XI 3.0/2.0.