Check out your Password validation => NO MaxLength(20) (only counts for UserName). If MaxLength has been set for the Hashed password, SQL will reject the given input. Also make sure in your migrations that Password column is correctly set up by EF-Core => 

            migrationBuilder.CreateTable(
                name: "Users",
                columns: table => new
                {
                    Id = table.Column<string>(nullable: false),
                    Username = table.Column<string>(maxLength: 20, nullable: false),
                    Email = table.Column<string>(nullable: false),
                    Password = table.Column<string>(nullable: false)
                },

User Class

        public User()
        {
            this.Id = Guid.NewGuid().ToString();
            this.Cards = new HashSet<UserCard>();
        }

        public string Id { get; set; }

        [Required]
        [MaxLength(20)]
        public string Username { get; set; }

        [Required]
        public string Email { get; set; }

        [Required]
        public string Password { get; set; }

        public virtual ICollection<UserCard> Cards { get; set; }
    }