I started to look into it, but I'm not sure what the problem is.
I can see that the createMailer function has been changed with Joomla 4.4.0 and that it is now deprecated and due to be removed in Joomla 6. But I don't see why that error would happen.
The callstack points, it seems to me, at some kind of incompatibility between the createMailer function of Factory in JROOT/libraries/src/Factory.php and the createMailer function of MailerFactory in JROOT/libraries/src/Mail/MailerFactory.php

Also 'Send Test Mail' fails:

#
#Date: 2023-10-19 06:43:38 UTC
#Software: Joomla! 4.4.0 Stable [ Pamoja ] 17-October-2023 16:00 GMT

#Fields: datetime priority clientip category message
2023-10-19T06:43:38+00:00 CRITICAL 2003:fe:4705:9d0b:a450:7dd4:404d:cf66 error Uncaught Throwable of type TypeError thrown with message "Joomla\CMS\Mail\MailerFactory::createMailer(): Return value must be of type Joomla\CMS\Mail\MailerInterface, Joomla\CMS\Mail\Mail returned". Stack trace: #0 [ROOT]/administrator/components/com_config/src/Model/ApplicationModel.php(1201): Joomla\CMS\Mail\MailerFactory->createMailer()
#1 [ROOT]/administrator/components/com_config/src/Controller/ApplicationController.php(270): Joomla\Component\Config\Administrator\Model\ApplicationModel->sendTestMail()
#2 [ROOT]/libraries/src/MVC/Controller/BaseController.php(693): Joomla\Component\Config\Administrator\Controller\ApplicationController->sendtestmail()
#3 [ROOT]/libraries/src/Dispatcher/ComponentDispatcher.php(143): Joomla\CMS\MVC\Controller\BaseController->execute()
#4 [ROOT]/libraries/src/Component/ComponentHelper.php(361): Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch()
#5 [ROOT]/libraries/src/Application/AdministratorApplication.php(143): Joomla\CMS\Component\ComponentHelper::renderComponent()
#6 [ROOT]/libraries/src/Application/AdministratorApplication.php(186): Joomla\CMS\Application\AdministratorApplication->dispatch()
#7 [ROOT]/libraries/src/Application/CMSApplication.php(293): Joomla\CMS\Application\AdministratorApplication->doExecute()
#8 [ROOT]/administrator/includes/app.php(61): Joomla\CMS\Application\CMSApplication->execute()
#9 [ROOT]/administrator/index.php(32): require_once('/homepages/19/d...')
#10 {main}

Which mailer are you using?

PHP-Mail

I have a feeling this is caused by a 3rd party plugin overriding the Joomla! Mail class (had it happen before, but now with type hinting it could lead to this error)

• Just to be sure, delete /administrator/cache/autoload_psr4.php in case there's some wrong mapping in there. It will get re-created on next page load.
• If you have 3rd party plugins try to unpublish them all (System - Plugins - under Filter Options select 'system' from '- Select Type -') and see if this fixes the issue
• Open /libraries/src/Mail/MailerFactory.php and look for the createMailer function: right after this line

$mailer = new Mail((bool) $configuration->get('throw_exceptions', true));

add these lines

$reflection = new \ReflectionClass($mailer);
var_dump($reflection->getName());
var_dump($reflection->getFilename());
die;

Take a look at the browser console - open the XHR request and show us the response.

Great, thank you!!!

I've disabled the plugin "AcyMailing - Override Joomla emails" and now everything works fine again!

closing as it is not from core