Problème page blanche quand ouverture page - plus Maj impossible car erreur

La moise continue
Heureusement j ai travaillé sur le site de secours sur le le net
Donc j ai suivi la procedure et installé la version pro, en clic glissé pour l install ( seule etait présente Builder CK de base apres verif ct bien la pro d 'installée.
je suis passé en version PhP 8 page blanche puis je n 'ai pas pu revenir en arrière j'ai eu ces messages
Deprecated: Joomla\Input\Input implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /homepages/2/d671679869/htdocs/maravalgiteBis/libraries/vendor/joomla/input/src/Input.php on line 41

Deprecated: Return type of Joomla\Input\Input::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /homepages/2/d671679869/htdocs/maravalgiteBis/libraries/vendor/joomla/input/src/Input.php on line 170

Deprecated: Joomla\CMS\Input\Input implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /homepages/2/d671679869/htdocs/maravalgiteBis/libraries/src/Input/Input.php on line 31

Warning: session_name(): Session name cannot be changed after headers have already been sent in /homepages/2/d671679869/htdocs/maravalgiteBis/libraries/joomla/session/handler/native.php on line 128

Deprecated: Joomla\CMS\Input\Cookie implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /homepages/2/d671679869/htdocs/maravalgiteBis/libraries/src/Input/Cookie.php on line 21

Warning: session_cache_limiter(): Session cache limiter cannot be changed after headers have already been sent in /homepages/2/d671679869/htdocs/maravalgiteBis/libraries/joomla/session/handler/native.php on line 235
Error: Failed to start application: Failed to start the session because headers have already been sent by "/homepages/2/d671679869/htdocs/maravalgiteBis/libraries/vendor/joomla/input/src/Input.php" at line 41.

Je suis repassé en version 7.4 , pareil je n 'ai pas pu revenir a l affichage du site meme en admisnistrator c bloqué sur cette page
voila voila

salut
tu peux me donner un accès admin et FTP ?
envoie moi ça par email
www.joomlack.fr/en/contact

pour info j'ai mis à jour la facture que tu peux télécharger depuis ton compte

ok, je te donne cela
notamment au site principal

Tu as une adresse mail ?

Je vais me servir de celle ou tu as envoyé la facture

je t'ai répondu
on continue par email, mais de mon côté j'accède à l'édition des pages et articles sans souci ...