Json.NET and RESTSharp - Error Converting Value

I have created an API that I am now attempting to access through another application. Even though I am using Newtonsoft.JSON to both serialize and deserialize the JSON, I am encountering a conversion error, with the following InnerException:

{"Could not cast or convert from System.String to System.Collections.Generic.List`1[MidamAPI.Models.ManagerDTO]."}

The following is the snippet that throws the error (in RESTFactory):

     public List<T> Execute<T>(String endPoint) where T : new() {
        RestClient client = new RestClient(BASE_URL);

        IRestRequest req = new RestRequest(endPoint);
        req.AddHeader("Authorization", "Bearer " + this._token);
        var response = client.Execute(req).Content;
        List<T> responseData = JsonConvert.DeserializeObject<List<T>>(response);

        return responseData;
    }

which is called here:

{
        RegisterViewModel vm = new RegisterViewModel();
        RESTFactory factory = new RESTFactory((String)Session["bearer"]);
        vm.availableManager = factory.Execute<ManagerDTO>("managers");
        vm.availableProperties = factory.Execute<PropertyDTO>("locations");
        vm.availableTrainers = factory.Execute<TrainerDTO>("trainers");

        return View(vm);
    }

The information is coming from the following API controller action (in a separate application):

    [LocalFilter]
[RoutePrefix("managers")]
public class ManagersController : ApiController
{
    UnitOfWork worker = new UnitOfWork();

       [Route("")]
    public string Get()
    {
        IEnumerable<ManagerDTO> dtoList = Mapper.Map<List<Manager>, List<ManagerDTO>>(worker.ManagerRepo.Get().ToList());

        foreach (ManagerDTO dto in dtoList)
        {
            Manager manager = worker.ManagerRepo.GetByID(dto.ID);
            dto.Stores = (Mapper.Map<IEnumerable<Property>, IEnumerable<PropertyDTO>>(manager.Properties)).ToList();
        }

        return JsonConvert.SerializeObject(dtoList);
    }

The DTOs are the same in both applications:

{
public class ManagerDTO
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public List<PropertyDTO> Stores { get; set; }
}




  public class PropertyDTO
{
    public int ID;
    public string ShortName { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string Zip { get; set; }
    public string Phone { get; set; }
    public string Lat { get; set; }
    public string Long { get; set; }
    public string GooglePlaceID { get; set; }
    public string PropertyNumber { get; set; }
    public int ManagerID { get; set; }
    public int TrainerID { get; set; }

    public string ManagerName { get; set; }
    public string ManagerEmail { get; set; }
    public string TrainerEmail { get; set; }

}

I tried using the json2csharp tool with the response, and everything seems fine to my eyes:

public class Store
{
    public int ID { get; set; }
    public string ShortName { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string Zip { get; set; }
    public string Phone { get; set; }
    public string Lat { get; set; }
    public string Long { get; set; }
    public string GooglePlaceID { get; set; }
    public string PropertyNumber { get; set; }
    public int ManagerID { get; set; }
    public int TrainerID { get; set; }
    public string ManagerName { get; set; }
    public string ManagerEmail { get; set; }
    public object TrainerEmail { get; set; }
}

public class RootObject
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public List<Store> Stores { get; set; }
}

Any advice would be appreciated.

EDIT:

The RegisterViewModel class public class RegisterViewModel {

    public RegistrationDTO regModel { get; set; }
    public List<ManagerDTO> availableManager { get; set; }
    public List<PropertyDTO> availableProperties { get; set; }
    public List<TrainerDTO> availableTrainers { get; set; }
}