I'm using the ASP.NET ReportViewer control in a application hosted in a load-balanced Web farm (using Microsoft Windows 2008 load balancing).

The ReportViewer is configured to connect to a sole back-end Report Server.

Often I get the dreaded rsExecutionNotFound '[unique id]' error.

It transpires that the Report Service is creating a session for each 'user'. The user in this scenario is my front-end Web application hosting the ReportViewer control.  
I'm using IIS7 on Windows 2008, so that the 'user' is actually the machine's network service (DOMAIN\MACHINE NAME$), as IIS7 is running in Integrated Pipeline mode.

When I hit WebServer 1, the user from the viewpoint of the Report Server is DOMAIN\WebServer1$. A session is created for this user.
If at some point the client browser hits the other node, the Report user becomes the other Web server - DOMAIN\WebServer2$. At this point I get the dreaded rsExecutionNotFound error.

The solution? Use a common user when your ReportViewer control connects to the Report Server. Don't forget to permission that user as a 'Browser' in the Report Manager!

ReportViewer.ServerReport.ReportServerCredentials = new ReportViewerCredentials(@"DOMAIN\MyUser", "MyPassword");

I spent ages Googling around trying to find the solution to this. Other people mention timeouts, etc, but in my case it was the user accessing the Web Service. Surely more people will come across this dreaded error in time! I hope this helps.

You'll need this code below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Microsoft.Reporting.WebForms;
using System.Runtime.InteropServices;
using System.Net;
using System.Security.Principal;

namespace Euro.UI.Framework
{
    [Serializable]
    public class ReportViewerCredentials : IReportServerCredentials
    {
        [DllImport("advapi32.dll", SetLastError = true)]
        public extern static bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword, int dwLogonType, int dwLogonProvider, ref IntPtr phToken);

        [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
        public extern static bool CloseHandle(IntPtr handle);

        [DllImport("advapi32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        public extern static bool DuplicateToken(IntPtr ExistingTokenHandle,
            int SECURITY_IMPERSONATION_LEVEL, ref IntPtr DuplicateTokenHandle);

        public ReportViewerCredentials()
        {
        }

        public ReportViewerCredentials(string username)
        {
            this.Username = username;
        }

        public ReportViewerCredentials(string username, string password)
        {
            this.Username = username;
            this.Password = password;
        }

        public ReportViewerCredentials(string username, string password, string domain)
        {
            this.Username = username;
            this.Password = password;
            this.Domain = domain;
        }

        public string Username
        {
            get
            {
                return this.username;
            }
            set
            {
                string username = value;
                if (username.Contains("\\"))
                {
                    this.domain = username.Substring(0, username.IndexOf("\\"));
                    this.username = username.Substring(username.IndexOf("\\") + 1);
                }
                else
                {
                    this.username = username;
                }
            }
        }
        private string username;



        public string Password
        {
            get
            {
                return this.password;
            }
            set
            {
                this.password = value;
            }
        }
        private string password;


        public string Domain
        {
            get
            {
                return this.domain;
            }
            set
            {
                this.domain = value;
            }
        }
        private string domain;


        #region IReportServerCredentials Members

        public bool GetBasicCredentials(out string basicUser, out string basicPassword, out string basicDomain)
        {
            basicUser = username;
            basicPassword = password;
            basicDomain = domain;
            return username != null && password != null && domain != null;
        }

        public bool GetFormsCredentials(out string formsUser, out string formsPassword, out string formsAuthority)
        {
            formsUser = username;
            formsPassword = password;
            formsAuthority = domain;
            return username != null && password != null && domain != null;

        }

        public bool GetFormsCredentials(out Cookie authCookie,
      out string user, out string password, out string authority)
        {
            authCookie = null;
            user = password = authority = null;
            return false;  // Not implemented
        }


        public WindowsIdentity ImpersonationUser
        {
            get
            {

                string[] args = new string[3] { this.Domain.ToString(), this.Username.ToString(), this.Password.ToString() };
                IntPtr tokenHandle = new IntPtr(0);
                IntPtr dupeTokenHandle = new IntPtr(0);

                const int LOGON32_PROVIDER_DEFAULT = 0;
                //This parameter causes LogonUser to create a primary token.
                const int LOGON32_LOGON_INTERACTIVE = 2;
                const int SecurityImpersonation = 2;

                tokenHandle = IntPtr.Zero;
                dupeTokenHandle = IntPtr.Zero;
                try
                {
                    // Call LogonUser to obtain an handle to an access token.
                    bool returnValue = LogonUser(args[1], args[0], args[2],
                        LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
                        ref tokenHandle);

                    if (false == returnValue)
                    {
                        Console.WriteLine("LogonUser failed with error code : {0}",
                            Marshal.GetLastWin32Error());
                        return null;
                    }

                    // Check the identity.
                    System.Diagnostics.Trace.WriteLine("Before impersonation: "
                        + WindowsIdentity.GetCurrent().Name);


                    bool retVal = DuplicateToken(tokenHandle, SecurityImpersonation, ref dupeTokenHandle);
                    if (false == retVal)
                    {
                        CloseHandle(tokenHandle);
                        Console.WriteLine("Exception in token duplication.");
                        return null;
                    }


                    // The token that is passed to the following constructor must
                    // be a primary token to impersonate.
                    WindowsIdentity newId = new WindowsIdentity(dupeTokenHandle);
                    WindowsImpersonationContext impersonatedUser = newId.Impersonate();


                    // Free the tokens.
                    if (tokenHandle != IntPtr.Zero)
                        CloseHandle(tokenHandle);
                    if (dupeTokenHandle != IntPtr.Zero)
                        CloseHandle(dupeTokenHandle);

                    // Check the identity.
                    System.Diagnostics.Trace.WriteLine("After impersonation: "
                        + WindowsIdentity.GetCurrent().Name);

                    return newId;

                }
                catch (Exception ex)
                {
                    Console.WriteLine("Exception occurred. " + ex.Message);
                }

                return null;
            }
        }

        public ICredentials NetworkCredentials
        {
            get
            {
                return null;  // Not using NetworkCredentials to authenticate.
            }
        }

        #endregion
    }
}