Stuck With Login Failed For User XxxxASPNET Error, Need A Hint Please

Hi,

I am stuck with that error when running my ASP.NET application on my production server for many weeks now and still don't have a complete solution to that problem.

I formerly posted that thread and read that other thread without success, so as I have made a few progress in delimiting the problem I am posting this new thread with the hope somebody will finally be able to help me.

To summarize:

1) I have developed an ASP.NET application,
- on a Windows XP Pro SP2 computer
- with VS studio 2005 Dev and included SQL 2005 engine
- this application works fine on the dev computer

2) I have deployed the application on a production server which is:
- a Windows 2000 SP 4 computer
- with SQL Express SP1 with Advance services installed

When I launch the prod application I have this error:
Login failed for user xxxxASPNET

I have found that I can bypass this error by doing the following:
1) manually attach my .mdf database with SQL Server Management Studio Express and specifying the field Attached as with myDatabaseName (in place of the default fullPhysicalPath/myDatabaseName.MDF) then detach it
2) change the auto generated connection string in web.config file from
<add name="ConnectionString" connectionString="Data Source=.SQLEXPRESS;AttachDbFilename=|DataDirectory|myDatabaseName.mdf;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient"/>
To
<add name="ConnectionString" connectionString="Data Source=.SQLEXPRESS;AttachDbFilename=|DataDirectory|myDatabaseName.mdf;User ID=sa;Password=mySAPassword;database=myDatabaseName" providerName="System.Data.SqlClient"/>

That would have been an acceptable substitution solution but unfortunately my
application also uses the new Login and Webparts features of VS 2005 which implies the automatic creation of an ASPNETDB.MDF database by VS.
As I have no mean (or at least don't know how) of modifying the associated Connection String for that database (it does not appear in web.config) I cannot bypass the problem for it and thus the error still appears when the application try to use this database.

Please I really need help on this matter, I can provide any other useful information as needed (logs, sources ...)