"Microsoft SQL Server Native Client 10.0" Hresult: 0x80040E4D Description: "Login failed for user 'UserName'."

I have VS2005 based .Net application that calls and execute SQL Server 2008 SSIS package and results to error.
It works perfectly when I execute package by VS2005 or by selecting package file. Error occurs only with .net application. The SQL account UserName has db_owner rights to the database. The username and password have been defined in web.config

Why this error occurs?

This is error:
-1071636471 SSIS Error Code DTS_E_OLEDBERROR. An OLE DB error has occurred. Error code: 0x80040E4D. An OLE DB record is available. Source: "Microsoft SQL Server Native Client 10.0" Hresult: 0x80040E4D Description: "Login failed for user 'UserName'.".
-1073573396 Failed to acquire connection "ServerName.DatabaseName.UserName". Connection may not be configured correctly or you may not have the right permissions on this connection.

This is the code:
Imports System.Data.SqlClient
Imports Microsoft.SqlServer.Dts.Runtime

      Dim pkgLocation As String
        Dim pkg As New Package
        Dim app As New Application
        Dim pkgResults As DTSExecResult

        pkgLocation = Package

        Try
            If Storage = "Server" Then
                If Trusted_Connection Then
                    pkg = app.LoadFromSqlServer(pkgLocation, DTS_Server, Nothing, Nothing, Nothing)
                Else
                    pkg = app.LoadFromSqlServer(pkgLocation, DTS_Server, DTS_UserId, DTS_Password, Nothing)
                End If
            Else
                pkg = app.LoadPackage(pkgLocation, Nothing)
            End If
        Catch ex As Exception
            lblMsg.Text = "<b>Package load did not succeed, error:</b> " & ex.Message
            pkg = Nothing
            app = Nothing
            Exit Sub
        End Try

        Try
            pkgResults = pkg.Execute()
        Catch ex As Exception
            lblMsg.Text = "<b>Package execution did not succeed, error:</b> " & ex.Message
        End Try