efcore: EF Core exception when i send multiple requests

I updated my project to ASP.NET Core 2.1 RTM. When I send multiple requests, i get this exception:

Exception has occurred: CLR/System.Data.SqlClient.SqlException
An exception of type 'System.Data.SqlClient.SqlException' occurred in Microsoft.EntityFrameworkCore.dll but was not handled in user code: 'Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.'
 Inner exceptions found, see $exception in variables window for more details.
 Innermost exception 	 System.ComponentModel.Win32Exception : Unknown error 258

In MyContext class:

public  class MyContext : DbContext, IUnitOfWork
{
    public MyContext(DbContextOptions<MyContext> options)
          : base(options) { }
    public virtual DbSet<MyEntity> MyEntity { get; set; }
}

In TestService class:

public class TestService : ITestService
    {
        private readonly IUnitOfWork _uow;
        private readonly DbSet<MyEntity> _myEntity;
        public LogRecentService(IUnitOfWork uow)
        {
            _uow = uow;
            _myEntity = _uow.Set<MyEntity>();
        }
        public void Add(MyEntity model)
        {
            _myEntity.Add(model);
            _uow.SaveChanges();
        }
    }

In Startup class:

services.AddScoped<IUnitOfWork, MyContext>();
services.AddEntityFrameworkSqlServer();
services.AddDbContextPool<MyContext>(options =>
            {
                string connectionString = Configuration.GetConnectionString("MyConnection");
                options.UseSqlServer(
                connectionString
                , serverDbContextOptionsBuilder =>
                {
                    var minutes = (int)TimeSpan.FromMinutes(3).TotalSeconds;
                    serverDbContextOptionsBuilder.CommandTimeout(minutes);
                    serverDbContextOptionsBuilder.EnableRetryOnFailure();
                });
            });
services.AddScoped<ITestService, TestService>();

Further technical details

EF Core version: 2.1 Operating system: Ubuntu 18.04 LTS