Asp.Net Web API Error: The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'

I would suggest Disable Proxy Creation only in the place where you don't need or is causing you trouble. You don't have to disable it globally you can just disable the current DB context via code...

    [HttpGet]
    [WithDbContextApi]
    public HttpResponseMessage Get(int take = 10, int skip = 0)
    {
        CurrentDbContext.Configuration.ProxyCreationEnabled = false;

        var lista = CurrentDbContext.PaymentTypes
            .OrderByDescending(x => x.Id)
            .Skip(skip)
            .Take(take)
            .ToList();

        var count = CurrentDbContext.PaymentTypes.Count();

        return Request.CreateResponse(HttpStatusCode.OK, new { PaymentTypes = lista, TotalCount = count });
    }

Here I only disabled the ProxyCreation in this method, because for every request there is a new DBContext created and therefore I only disabled the ProxyCreation for this case . Hope it helps

if you have navigation properties and you do not want make them non virtual, you should using JSON.NET and change configuration in App_Start to using JSON not XML!
after install JSON.NET From NuGet, insert this code in WebApiConfig.cs in Register method

var json = config.Formatters.JsonFormatter;
json.SerializerSettings.PreserveReferencesHandling = Newtonsoft.Json.PreserveReferencesHandling.Objects;
config.Formatters.Remove(config.Formatters.XmlFormatter);

If you have navigation properties make them non virtual. Mapping will still work but it prevents the creation of Dynamic Proxy entities which cannot be serialized.]

Not having lazy loading is fine in a WebApi as you don't have a persistent connection and you ran a .ToList() anyway.