Update auf Shopware 6.5.2.1 schlägt fehl, ClassMetadata::$classDiscriminatorMapping must not be accessed before initialization

Hallo zusammen,

ich wollte heute einmal meinen Shop auf die neuste Version bringen.
Leider ist dies mit der folgenden Meldung fehlgeschlagen.

19:18:03 DEBUG     [console] Command "system:update:finish --no-interaction" exited with code "1"
[
  "command" => "system:update:finish --no-interaction",
  "code" => 1
]

In ClassMetadata.php line 90:
                                                                               
  [Error]                                                                      
  Typed property Symfony\Component\Serializer\Mapping\ClassMetadata::$classDi  
  scriminatorMapping must not be accessed before initialization                
                                                                               

Exception trace:
  at /vendor/symfony/serializer/Mapping/ClassMetadata.php:90
 Symfony\Component\Serializer\Mapping\ClassMetadata->getClassDiscriminatorMapping() at /vendor/symfony/serializer/Mapping/ClassDiscriminatorFromClassMetadata.php:42
 Symfony\Component\Serializer\Mapping\ClassDiscriminatorFromClassMetadata->getMappingForMappedObject() at /vendor/symfony/serializer/Normalizer/AbstractObjectNormalizer.php:272
 Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer->getAttributes() at /vendor/symfony/serializer/Normalizer/AbstractObjectNormalizer.php:167
 Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer->normalize() at /vendor/symfony/serializer/Serializer.php:159
 Symfony\Component\Serializer\Serializer->normalize() at /vendor/symfony/serializer/Serializer.php:177
 Symfony\Component\Serializer\Serializer->normalize() at /vendor/symfony/serializer/Serializer.php:138
 Symfony\Component\Serializer\Serializer->serialize() at /vendor/symfony/messenger/Transport/Serialization/Serializer.php:143
 Symfony\Component\Messenger\Transport\Serialization\Serializer->encodeStamps() at /vendor/symfony/messenger/Transport/Serialization/Serializer.php:104
 Symfony\Component\Messenger\Transport\Serialization\Serializer->encode() at /vendor/symfony/doctrine-messenger/Transport/DoctrineSender.php:39
 Symfony\Component\Messenger\Bridge\Doctrine\Transport\DoctrineSender->send() at /vendor/symfony/doctrine-messenger/Transport/DoctrineTransport.php:72
 Symfony\Component\Messenger\Bridge\Doctrine\Transport\DoctrineTransport->send() at /vendor/symfony/messenger/Middleware/SendMessageMiddleware.php:68
 Symfony\Component\Messenger\Middleware\SendMessageMiddleware->handle() at /vendor/symfony/messenger/Middleware/FailedMessageProcessingMiddleware.php:34
 Symfony\Component\Messenger\Middleware\FailedMessageProcessingMiddleware->handle() at /vendor/symfony/messenger/Middleware/DispatchAfterCurrentBusMiddleware.php:68
 Symfony\Component\Messenger\Middleware\DispatchAfterCurrentBusMiddleware->handle() at /vendor/symfony/messenger/Middleware/RejectRedeliveredMessageMiddleware.php:41
 Symfony\Component\Messenger\Middleware\RejectRedeliveredMessageMiddleware->handle() at /vendor/symfony/messenger/Middleware/AddBusNameStampMiddleware.php:37
 Symfony\Component\Messenger\Middleware\AddBusNameStampMiddleware->handle() at /vendor/symfony/messenger/MessageBus.php:70
 Symfony\Component\Messenger\MessageBus->dispatch() at /vendor/shopware/core/Content/Media/File/FileSaver.php:112
 Shopware\Core\Content\Media\File\FileSaver->persistFileToMedia() at /vendor/shopware/storefront/Theme/ThemeLifecycleService.php:397
 Shopware\Storefront\Theme\ThemeLifecycleService->updateMediaInConfiguration() at /vendor/shopware/storefront/Theme/ThemeLifecycleService.php:78
 Shopware\Storefront\Theme\ThemeLifecycleService->refreshTheme() at /vendor/shopware/storefront/Theme/ThemeLifecycleService.php:55
 Shopware\Storefront\Theme\ThemeLifecycleService->refreshThemes() at /vendor/shopware/storefront/Theme/Subscriber/UpdateSubscriber.php:51
 Shopware\Storefront\Theme\Subscriber\UpdateSubscriber->updateFinished() at /vendor/symfony/event-dispatcher/EventDispatcher.php:260
 Symfony\Component\EventDispatcher\EventDispatcher::Symfony\Component\EventDispatcher\{closure}() at /vendor/symfony/event-dispatcher/EventDispatcher.php:220
 Symfony\Component\EventDispatcher\EventDispatcher->callListeners() at /vendor/symfony/event-dispatcher/EventDispatcher.php:56
 Symfony\Component\EventDispatcher\EventDispatcher->dispatch() at /vendor/shopware/core/Content/Flow/Dispatching/FlowDispatcher.php:47
 Shopware\Core\Content\Flow\Dispatching\FlowDispatcher->dispatch() at /vendor/shopware/core/Framework/Webhook/WebhookDispatcher.php:67
 Shopware\Core\Framework\Webhook\WebhookDispatcher->dispatch() at /vendor/shopware/core/Framework/Event/NestedEventDispatcher.php:31
 Shopware\Core\Framework\Event\NestedEventDispatcher->dispatch() at /vendor/shopware/core/Maintenance/System/Command/SystemUpdateFinishCommand.php:97
 Shopware\Core\Maintenance\System\Command\SystemUpdateFinishCommand->execute() at /vendor/symfony/console/Command/Command.php:326
 Symfony\Component\Console\Command\Command->run() at /vendor/symfony/console/Application.php:1081
 Symfony\Component\Console\Application->doRunCommand() at /vendor/symfony/framework-bundle/Console/Application.php:91
 Symfony\Bundle\FrameworkBundle\Console\Application->doRunCommand() at /vendor/symfony/console/Application.php:320
 Symfony\Component\Console\Application->doRun() at /vendor/symfony/framework-bundle/Console/Application.php:80
 Symfony\Bundle\FrameworkBundle\Console\Application->doRun() at /vendor/symfony/console/Application.php:174
 Symfony\Component\Console\Application->run() at /vendor/symfony/runtime/Runner/Symfony/ConsoleApplicationRunner.php:54
 Symfony\Component\Runtime\Runner\Symfony\ConsoleApplicationRunner->run() at /vendor/autoload_runtime.php:29
 require_once() at /bin/console:15

Shopware 6.5.0.0 war vor dem Update installiert.
Die Zwischenschritte bis 6.5.2.0 habe problemlos geklappt.
Beim Update auf die Version 6.5.2.1 trat dann der Fehler auf.
Die restlichen Meldungen im Log sind in Ordnung.
Nur dieser eine Critical Error verhindert das vollständige Update.

Grund für das späte Update ist, dass im Backend selbst keine Updates kamen.
Im Menüpunkt „System → Shopware-Aktualisierungen“ stand immer, dass ich auf dem neusten Stand bin.
Laut Changelog gab es aber bereits mehrere Updates seit 6.5.0.0.
Daher habe ich dann ein Update über den Installer gemacht, den ich im Changelog herunterladen konnte. „shopware-installer.phar.php“

Vielleicht hat einer von euch eine Idee.
Danke schon einmal und noch einen schönen Abend.