Problems after Joomla 4.4 upgrade

-- url of the page with the problem -- : www.wunderwerkstatt.eu/downloads.html
-- HikaShop version -- : 5.0.0
-- Joomla version -- : 4.4
-- PHP version -- : 8.1
-- Browser(s) name and version -- : Safari 15.6.1 und Firefox 118.02
-- Error-message(debug-mod must be tuned on) -- : 0 - Joomla\CMS\Mail\MailerFactory::createMailer(): Return value must be of type Joomla\CMS\Mail\MailerInterface, Joomla\CMS\Mail\Mail returned
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Joomla\CMS\Mail\MailerFactory::createMailer(): Return value must be of type Joomla\CMS\Mail\MailerInterface, Joomla\CMS\Mail\Mail returned

Call stack
# Function Location
1 () JROOT/libraries/src/Mail/MailerFactory.php:106
2 Joomla\CMS\Mail\MailerFactory->createMailer() JROOT/libraries/src/Factory.php:708
3 Joomla\CMS\Factory::createMailer() JROOT/libraries/src/Factory.php:490
4 Joomla\CMS\Factory::getMailer() JROOT/components/com_hikashop/controllers/product.php:257
5 productController->add_waitlist() JROOT/libraries/src/MVC/Controller/BaseController.php:693
6 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/administrator/components/com_hikashop/helpers/helper.php:2535
7 hikashopController->execute() JROOT/components/com_hikashop/hikashop.php:88
8 require_once() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:71
9 Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:73
10 Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:361
11 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/SiteApplication.php:208
12 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/SiteApplication.php:249
13 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:293
14 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/app.php:61
15 require_once() JROOT/index.php:32

Hi everyone. I'm really happy with using hikashop business (5.0 at the moment) for some years now. but since my Joomla 4.4 update today at first glance everything runs smoothly.
When someone tries to buy a product everything works fine until checkout last step when you press buy, there is an error message:
0 - Joomla\CMS\Mail\MailerFactory::createMailer(): Return value must be of type Joomla\CMS\Mail\MailerInterface, Joomla\CMS\Mail\Mail returned.
But I can see the order in backend. But I get no mail, that an order was created. And I can’t change the Order-Status in backend. I get the same errormessage
When someone tries to get subscribe to a waitlist the same error occurs, when you click on send.
thanks in Advance

Thanks Nicolas

i tested on my test server, but I didn't test so deep. almost everything is working. Acymailling sends mails but hikashop not. so are all functions which require an automated mail "broken".

I just tested it - and I get this error message

Beim Abrufen von JSON-Daten wurde ein HTTP-Statuscode 500 zurückgegeben.

Hope this helps. I'm back to my backup-version 4.3 - and the "bad" version is on test server - I gave me this message

Wow thanks for keeping track. Yes after disabling this plugin everything works as it should. I will report to Acymailing.

Thanks again!

Is it now OK to upgrade to Joomla 4.4.0 as I havn't done it yet?

yes - the Problem was acymailing plugin mentioned above - with the new acymailling upgrade the Joomla 4.4 runs smoothly