Web API error - Server Error in '/api' Application

After publishing Web API to IIS, which is a child of an AngularJs IIS site, I can reach 'https://localhost/api' and see all endpoints; but when I try to reach some specific endpoint with a GET request, I get an error

Server Error in '/' Application

public class WebApiApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();
        GlobalConfiguration.Configure(WebApiConfig.Register);
        FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
        RouteConfig.RegisterRoutes(RouteTable.Routes);
        BundleConfig.RegisterBundles(BundleTable.Bundles);
    }
}

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.SuppressDefaultHostAuthentication();
        config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

[RoutePrefix("api/branches")]
public class BranchesController : ApiBaseController
{
    [HttpGet]
    [Route("getBranches")]
    public async Task<JsonResult<List<BranchDto>>> GetActiveBranches()
    {
        var branches = new List<BranchDto>();

        var models = await WarehouseUnitOfWork.BranchRepository.GetActiveBranches();
        if (models != null && models.Count > 0)
        {
            branches.AddRange(AutoMapper.Mapper.Map<List<Branch>, List<BranchDto>>(models));
        }

        return Json(branches, new JsonSerializerSettings
        {
            ContractResolver = new WarehouseCustomContractResolver()
        });
    }
}

Any ideas how to solve this?