Have you checked on your server to see if you're using the right php?

Chatgpt means

The provided error message originates from a PHP application, likely within a Joomla!-based Content Management System (CMS). Here is a brief analysis of the message:

The error occurs in the file offline.php, located in the directory JROOT/templates/themis/, at line 65.
The error is triggered by a require() statement in the file HtmlDocument.php, which is located in the directory JROOT/libraries/src/Document/. Specifically, at line 731 of this file.
The require() statement is part of the _loadTemplate() method defined in the HtmlDocument class (line 785).
This method, in turn, calls the _fetchTemplate() method (line 622).
The error occurs during the parsing of the HTML document, initiated by the parse() method in the HtmlDocument class (line 1057).
The actual invocation of the parse() method happens in the CMSApplication class (Content Management System Application) in the file CMSApplication.php (line 311).
The application then attempts to render (line 732), and finally, the execute() method is called in the file app.php (line 58).
The entire process begins with the invocation of the index.php file in JROOT (line 32).

The exact cause of the error can vary, and it's important to note that this is only a general analysis based on the provided error message. To resolve the issue, it is necessary to inspect the code in the mentioned files and determine the specific cause of the error. It could be a missing file path, a syntax error, or another issue. Dependencies might also be incorrectly installed. Reviewing the relevant code lines in the mentioned files could provide further insights.

Unfortunately I am not a programmer.

Hallo Rashida,

I tried both. Even I deleted the cache in the Joomla system and in the browser. Nothing changed.

Edit: Now at 8.0 ... it comes

Sorry, your PHP version is not supported

Your host needs to use PHP version 8.1.0 or newer to run this version of Joomla!

--> here doesnt work the backend

With 8.1 it coms:

0
Class "Html" not found

--> Here works the backend.

Here the screenshot from the mistakes

https://prnt.sc/G2YMg-PdveYV