There is an issue with the script in PHP 8 which is causing the calendar to not display properly.
Here is a quick fix for existing versions (no need to update):
In ac-includes > common.inc.php and change line 16 (latest version) from this:
//error_reporting(E_ALL ^ E_NOTICE);to this:
error_reporting(0);
This code turns off PHP error reporting just for the calendar script.