Getting SqlException while await _context.SaveChangesAsync();

I m encrypting some sensitive data in my SQL db (i.e. SSN). I have have my encryption key saved in a table and with each encryption/Decryption request they key get called from the table. 

    public static class DbEncryptionHandler
    {
        public static string StaticDecrypt(string cipherText, ApplicationDbContext _context)
        {
            try
            {
                if (!string.IsNullOrWhiteSpace(cipherText))
                {


                    List<EncKeys> Keys = new List<EncKeys>();
                    Keys = _context.EncKeys.FromSql("EncKeysSP").ToList();
                    string EncryptionKey = Keys[0].StaticKey;
                    cipherText = cipherText.Replace(" ", "+");
                    byte[] cipherBytes = Convert.FromBase64String(cipherText);
                    using (Aes encryptor = Aes.Create())
                    {
                        Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
                        encryptor.Key = pdb.GetBytes(32);
                        encryptor.IV = pdb.GetBytes(16);
                        using (MemoryStream ms = new MemoryStream())
                        {
                            using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                            {
                                cs.Write(cipherBytes, 0, cipherBytes.Length);
                                cs.Close();
                            }
                            cipherText = Encoding.Unicode.GetString(ms.ToArray());
                        }
                    }
                }
                else
                {

                    cipherText = null;

                }
            }
            catch (Exception ex)
            {
                throw;
            }

            return cipherText;
        }
}

this is the controller

 public async Task<IActionResult> Edit(long? id)
        {

            if (id == null)
            {
                return NotFound();
            }

            var patReg = await _context.PatReg.SingleOrDefaultAsync(m => m.FileId == id);
            patReg.SNN = DbEncryptionHandler.StaticDecrypt(patReg.SNN, _context);
            if (patReg == null)
            {
                return NotFound();
            }
            return View(patReg);
        }

        // POST: PatReg/Edit/5
        // To protect from overposting attacks, please enable the specific properties you want to bind to, for
        // more details see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Edit(long id, [Bind("FileId,FName,MName,LName,Dob,GenrId,StasId,NatyId,MarsId,CouyId,StaeId,CityId,OccnId,Email,SNN,PassNo,MobNo,LLine,MAdds,StrtNo,SDirn,AptNo,Locy,ALevl,PCode,Couy,ProeId")] PatReg patReg)
        {
            if (id != patReg.FileId)
            {
                return NotFound();
            }

            if (ModelState.IsValid)
            {
                try
                {
                    patReg.SNN = DbEncryptionHandler.StaticEncrypt(patReg.SNN, _context);

                    _context.Update(patReg);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!PatRegExists(patReg.FileId))
                    {
                        return NotFound();
                    }
                    else
                    {
                        throw;
                    }
                }
                return RedirectToAction(nameof(Index));
            }
            return View(patReg);
        }

I m getting this error while saving

SqlException: New transaction is not allowed because there are other threads running in the session.
System.Data.SqlClient.SqlConnection.OnError(SqlException exception, bool breakConnection, Action<Action> wrapCloseInAction)


                    await _context.SaveChangesAsync();
               

what am i doing wrong here?