Hi,
Thank you for the access.
When looking at the HTML of the popup, I didn't see the HTML editor for the email preview in the popup (which appears if you activate the user notification yes/no). So I displayed again the popup after activating temporarily the debug and error reporting in your Joomla configuration and I got that error:
Fatal error: Array and string offset access syntax with curly braces is no longer supported in public_html/components/com_jce/editor/libraries/classes/browser.php on line 270
As you can see the error comes from JCE. The error message means that your version of JCE doesn't support PHP8.
In fact, you have the same issue when editing a product, or editing a Joomla article (normal since they also use the HTML editor).
So I suppose you didn't update JCE. Do it and it should fix the problem, or you can switch to TinyMCE.