I understand PHP 8.1 RC3 is not officially supported yet but decided to report this anyway. It worked fine with 4.4.1 until I upgraded to 4.5. Unfortunately downgrade doesn't work (database has already been upgraded) so I did some debugging to figure out a workaround.

I tripple checked that Matomo 4.5 was properly installed with a diff -qr between the zip and my installation.

PROBLEM:

PHP Fatal error: During inheritance of IteratorAggregate: Uncaught Error: Class "Piwik\Session\SessionNamespace" not found in /var/www/.../core/Notification/Manager.php:203 Stack trace:#0 /var/www/.../core/Notification/Manager.php(147): Piwik\Notification\Manager::getSession()

WORKAROUND:

File: /var/www/.../plugins/Monolog/Handler/WebNotificationHandler.php
Comment out: Manager::notify(Common::getRandomString(), $notification);