Sorry i'm French, so giving details in english is not really easy.
Lots of things, like backgrounds, margins, font size and bold, template, widgetkit, slideshow ....
everythings turns bad when update is applied
Do you use YooTheme Widgetkit or YooTheme Zoo? I know you said Widgetkit above. So it may be the same issue I'm having because it seems those two components are causing a MenuHelper error.
This was fixed over email, I made changes to the Menu.php which removed Joomla 4 Compatibility layer for now but it will be back with a fix in the final release.
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