Got the solution ! (Edit: in fact not : see my post below)

It's the strored precedure AddPortalInfo that is not changed by the update. Here is the old procedure to be changed :

CREATE procedure dbo.AddPortalInfo
@PortalName         nvarchar(128),
@Currency           char(3),
@FirstName          nvarchar(100),
@LastName           nvarchar(100),
@Username           nvarchar(100),
@Password           nvarchar(50),
@Email              nvarchar(100),
@ExpiryDate         datetime,
@HostFee            money,
@HostSpace          int,
@SiteLogHistory     int,
@HomeDirectory varchar(100)

as

DECLARE @PortalID int

insert into dbo.Portals (
  PortalName,
  ExpiryDate,
  UserRegistration,
  BannerAdvertising,
  Currency,
  HostFee,
  HostSpace,
  Description,
  KeyWords,
  SiteLogHistory,
  HomeDirectory
)
values (
  @PortalName,
  @ExpiryDate,
  0,
  0,
  @Currency,
  @HostFee,
  @HostSpace,
  @PortalName,
  @PortalName,
  @SiteLogHistory,
  @HomeDirectory
)

SET @PortalID = SCOPE_IDENTITY()

IF @HomeDirectory = ''
BEGIN
 UPDATE dbo.Portals SET HomeDirectory = 'Portals/' + convert(varchar(10), @PortalID) WHERE PortalID = @PortalID
END

SELECT @PortalID

Here is the sql query to apply to make the correction :

ALTER PROCEDURE dbo.[AddPortalInfo]
 @PortalName         nvarchar(128),
 @Currency           char(3),
 @ExpiryDate         datetime,
 @HostFee            money,
 @HostSpace          int,
 @SiteLogHistory     int,
 @HomeDirectory  varchar(100)

as
DECLARE @PortalID int

insert into Portals (
  PortalName,
  ExpiryDate,
  UserRegistration,
  BannerAdvertising,
  Currency,
  HostFee,
  HostSpace,
  Description,
  KeyWords,
  SiteLogHistory,
  HomeDirectory
)
values (
  @PortalName,
  @ExpiryDate,
  0,
  0,
  @Currency,
  @HostFee,
  @HostSpace,
  @PortalName,
  @PortalName,
  @SiteLogHistory,
  @HomeDirectory
)

SET @PortalID = SCOPE_IDENTITY()

IF @HomeDirectory = ''
BEGIN
 UPDATE Portals SET HomeDirectory = 'Portals/' + convert(varchar(10), @PortalID) WHERE PortalID = @PortalID
END

SELECT @PortalID
GO

I hope this might help someone else.

Regards,
Fabian

Fabian,

I have not encountered that issue with a new portal (did not make an upgrade from 4.0.3 yet), are you sure, that you made all necessary changes to web.config, Have you checked the upgrade logs in the sqldataprovider folder?