Hi,
It's a bug linked to PHP8.
However, the error message is a generic one.
For example, someone else contacted us for that same error message a month ago and the issue was in the language file of code mirror :
www.hikashop.com/forum/content-creation/...th-php-8/342137.html
Similarly to what that other user did, the first thing we need is that you activate the "debug system" setting of your Joomla configuration page and try again. It should display an error message with a callstack which will allow us to understand what's going on so that we can tell you what to do exactly.