Perte de session avec MSIE6 mais pas avec Mozilla ?

Bonjour à tous,
Je suis en train de redévelopper "proprement" mon pitit intranet, et
donc de nettoyer la gestion de mes sessions. Problème : tout marche bien
sous Firefox, mais IE6 perd la session dès que je change de page. Le
plus fun : IE 5 marche tout bien...

Sous PHP 4.1.2 (autre sreveur), ni Mozilla ni IE ne se comportent comme
je le veux.

Pourtant les php.ini sont (à priori) configurés pareil.

Des idées ?

Quelques détails ci-dessous.

Merci d'avance, je m'arrache les cheveux !

Damien

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Quelques infos :

Création de la session dans admin.php :
session_start();
$_SESSION['UID'] = $Utilisateur['id'];

Sur la page suivante (fiche.php) :
session_start();
if (!isset($_SESSION['UID']))
{
header("Location:erreur.php?erreur=1");
exit;
}

Extrait de PHP.ini :
[Session]
; Handler used to store/retrieve data.
session.save_handler = files

; Argument passed to save_handler. In the case of files, this is the path
; where data files are stored. Note: Windows users have to change this
; variable in order to use PHP's session functions.
; As of PHP 4.0.1, you can define the path as:
; session.save_path = "N;/path"
; where N is an integer. Instead of storing all the session files in
; /path, what this will do is use subdirectories N-levels deep, and
; store the session data in those directories. This is useful if you
; or your OS have problems with lots of files in one directory, and is
; a more efficient layout for servers that handle lots of sessions.
; NOTE 1: PHP will not create this directory structure automatically.
; You can use the script in the ext/session dir for that purpose.
; NOTE 2: See the section on garbage collection below if you choose to
; use subdirectories for session storage
session.save_path = "C:\PHP\sessiondata"

; Whether to use cookies.
session.use_cookies = 1

; This option enables administrators to make their users invulnerable to
; attacks which involve passing session ids in URLs; defaults to 0.
; session.use_only_cookies = 1

; Name of the session (used as cookie name).
session.name = PHPSESSID

; Initialize session on request startup.
session.auto_start = 0

; Lifetime in seconds of cookie or, if 0, until browser is restarted.
session.cookie_lifetime = 0

; The path for which the cookie is valid.
session.cookie_path = /

; The domain for which the cookie is valid.
session.cookie_domain =

; Handler used to serialize data. php is the standard serializer of PHP.
session.serialize_handler = php

; Define the probability that the 'garbage collection' process is started
; on every session initialization.
; The probability is calculated by using gc_probability/gc_divisor,
; e.g. 1/100 means there is a 1% chance that the GC process starts
; on each request.

session.gc_probability = 1
session.gc_divisor = 100

; After this number of seconds, stored data will be seen as 'garbage' and
; cleaned up by the garbage collection process.
session.gc_maxlifetime = 1440

; NOTE: If you are using the subdirectory option for storing session files
; (see session.save_path above), then garbage collection does *not*
; happen automatically. You will need to do your own garbage
; collection through a shell script, cron entry, or some other
method.
; For example, the following script would is the equivalent of
; setting session.gc_maxlifetime to 1440 (1440 seconds = 24 minutes):
; cd /path/to/sessions; find -cmin +24 | xargs rm

; PHP 4.2 and less have an undocumented feature/bug that allows you to
; to initialize a session variable in the global scope, albeit
register_globals
; is disabled. PHP 4.3 and later will warn you, if this feature is used.
; You can disable the feature and the warning seperately. At this time,
; the warning is only displayed, if bug_compat_42 is enabled.

session.bug_compat_42 = 1
session.bug_compat_warn = 1

; Check HTTP Referer to invalidate externally stored URLs containing ids.
; HTTP_REFERER has to contain this substring for the session to be
; considered as valid.
session.referer_check =

; How many bytes to read from the file.
session.entropy_length = 0

; Specified here to create the session id.
session.entropy_file =

;session.entropy_length = 16

;session.entropy_file = /dev/urandom

; Set to {nocache,private,public,} to determine HTTP caching aspects
; or leave this empty to avoid sending anti-caching headers.
session.cache_limiter = nocache

; Document expires after n minutes.
session.cache_expire = 180

; trans sid support is disabled by default.
; Use of trans sid may risk your users security.
; Use this option with caution.
; - User may send URL contains active session ID
; to other person via. email/irc/etc.
; - URL that contains active session ID may be stored
; in publically accessible computer.
; - User may access your site with the same session ID
; always using URL stored in browser's history or bookmarks.
session.use_trans_sid = 0

; The URL rewriter will look for URLs in a defined set of HTML tags.
; form/fieldset are special; if you include them here, the rewriter will
; add a hidden <input> field with the info which is otherwise appended
; to URLs. If you want XHTML conformity, remove the form entry.
; Note that all valid entries require a "=", even if no value follows.
url_rewriter.tags = "a=href,area=href,frame=src,input=src,form=,fieldset="