This Error:
The ‘ObjectContent`1’ type failed to serialize the response body for content type ‘application/xml; charset=utf-8’.

can be fixed by adding one line of code to your data model’s context class

public partial class YOURENTITYCLASS: DbContext
{
public YOURENTITY(){
base.Configuration.ProxyCreationEnabled = false;
}
...


Many times it triggers when you have foreign key constraints within your Entity Data Model.