diff --git a/phpstan.neon b/phpstan.neon
index 6b16c54b..46c6eae8 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -9,6 +9,11 @@ parameters:
         - '#generic class Shopware\\Core\\Framework\\Struct\\ArrayStruct does not specify its types#'
         - '#generic class Shopware\\Core\\Framework\\Struct\\ArrayStruct but does not specify its types#'
 
+        # ignored errors for backward compatibility (SW 6.5.3)
+        - '#Call to method getOrderTransactionId\(\) on an unknown class Shopware\\Core\\Checkout\\Payment\\Exception\\SyncPaymentProcessException#'
+        - '#Call to function method_exists\(\) with .*Shopware\\Core\\Framework\\Routing\\RoutingException.* and .* will always evaluate to false#'
+        - '#Call to method getOrderTransactionId\(\) on an unknown class Shopware\\Core\\Checkout\\Payment\\PaymentException#'
+
         # NEXT-29041 - Shopware issue: https://github.com/shopware/shopware/blob/v6.6.0.0-rc2/phpstan.neon.dist#L221
         - '#.* generic class Shopware\\Core\\Framework\\DataAbstractionLayer\\EntityRepository.*not specify its types: TEntityCollection#'
         - '#\(\) should return .*\|null but returns Shopware\\Core\\Framework\\DataAbstractionLayer\\Entity\|null#'
diff --git a/src/Components/OrderManagement/Controller/ProductPanel.php b/src/Components/OrderManagement/Controller/ProductPanel.php
index 920deaf5..9929ceff 100644
--- a/src/Components/OrderManagement/Controller/ProductPanel.php
+++ b/src/Components/OrderManagement/Controller/ProductPanel.php
@@ -25,10 +25,12 @@
 use Ratepay\RpayPayments\Components\RatepayApi\Service\Request\PaymentDeliverService;
 use Ratepay\RpayPayments\Components\RatepayApi\Service\Request\PaymentReturnService;
 use Ratepay\RpayPayments\Util\CriteriaHelper;
+use RuntimeException;
 use Shopware\Core\Checkout\Order\OrderEntity;
 use Shopware\Core\Framework\Context;
 use Shopware\Core\Framework\DataAbstractionLayer\EntityRepository;
 use Shopware\Core\Framework\DataAbstractionLayer\Search\Criteria;
+use Shopware\Core\Framework\Routing\Exception\InvalidRequestParameterException;
 use Shopware\Core\Framework\Routing\RoutingException;
 use Shopware\Core\Framework\Validation\DataValidationDefinition;
 use Shopware\Core\Framework\Validation\DataValidator;
@@ -181,7 +183,15 @@ protected function processModify(Request $request, Context $context, string $ope
         if ($order instanceof OrderEntity) {
             $params = $request->request->all();
             if (!isset($params['items']) || !is_array($params['items'])) {
-                throw RoutingException::invalidRequestParameter('items');
+                if (class_exists(RoutingException::class) && method_exists(RoutingException::class, 'invalidRequestParameter')) {
+                    throw RoutingException::invalidRequestParameter('items');
+                } elseif (class_exists(InvalidRequestParameterException::class)) {
+                    // required for shopware == 6.5.0.0
+                    throw new InvalidRequestParameterException('items');
+                }
+
+                // should never occur - just to be safe
+                throw new RuntimeException('Invalid request parameter: items');
             }
 
             $items = [];
diff --git a/src/Components/PaymentHandler/AbstractPaymentHandler.php b/src/Components/PaymentHandler/AbstractPaymentHandler.php
index bf9d6e6f..7c19af0e 100644
--- a/src/Components/PaymentHandler/AbstractPaymentHandler.php
+++ b/src/Components/PaymentHandler/AbstractPaymentHandler.php
@@ -28,9 +28,11 @@
 use Ratepay\RpayPayments\Components\RatepayApi\Service\Request\PaymentRequestService;
 use Ratepay\RpayPayments\Exception\RatepayException;
 use Ratepay\RpayPayments\Util\CriteriaHelper;
+use RuntimeException;
 use Shopware\Core\Checkout\Order\OrderEntity;
 use Shopware\Core\Checkout\Payment\Cart\PaymentHandler\SynchronousPaymentHandlerInterface;
 use Shopware\Core\Checkout\Payment\Cart\SyncPaymentTransactionStruct;
+use Shopware\Core\Checkout\Payment\Exception\SyncPaymentProcessException;
 use Shopware\Core\Checkout\Payment\PaymentException;
 use Shopware\Core\Framework\Context;
 use Shopware\Core\Framework\DataAbstractionLayer\Entity;
@@ -42,6 +44,7 @@
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Session\Session;
+use Throwable;
 
 abstract class AbstractPaymentHandler implements SynchronousPaymentHandlerInterface
 {
@@ -71,10 +74,7 @@ public function pay(SyncPaymentTransactionStruct $transaction, RequestDataBag $d
         $order = $this->getOrderWithAssociations($transaction->getOrder(), $salesChannelContext->getContext());
 
         if (!$order instanceof OrderEntity || $ratepayData->count() === 0) {
-            throw PaymentException::syncProcessInterrupted(
-                $transaction->getOrderTransaction()->getId(),
-                'unknown error during payment'
-            );
+            throw $this->syncProcessInterrupted($transaction->getOrderTransaction()->getId(), 'unknown error during payment');
         }
 
         try {
@@ -137,11 +137,7 @@ public function pay(SyncPaymentTransactionStruct $transaction, RequestDataBag $d
                 $session->getFlashBag()->add(StorefrontController::DANGER, $ratepayException->getMessage());
             }
 
-            throw PaymentException::syncProcessInterrupted(
-                $transaction->getOrderTransaction()->getId(),
-                $ratepayException->getMessage(),
-                $ratepayException
-            );
+            throw $this->syncProcessInterrupted($transaction->getOrderTransaction()->getId(), $ratepayException->getMessage(), $ratepayException);
         }
     }
 
@@ -187,4 +183,17 @@ protected function getOrderWithAssociations(OrderEntity $order, Context $context
     {
         return $this->orderRepository->search(CriteriaHelper::getCriteriaForOrder($order->getId()), $context)->first();
     }
+
+    private function syncProcessInterrupted(string $orderTransactionId, string $errorMessage, ?Throwable $e = null): Throwable
+    {
+        if (class_exists(PaymentException::class)) {
+            return PaymentException::syncProcessInterrupted($orderTransactionId, $errorMessage, $e);
+        } elseif (class_exists(SyncPaymentProcessException::class)) {
+            // required for shopware version <= 6.5.3
+            return new SyncPaymentProcessException($orderTransactionId, $errorMessage, $e); // @phpstan-ignore-line
+        }
+
+        // should never occur - just to be safe
+        return new RuntimeException('payment interrupted: ' . $errorMessage, 0, $e);
+    }
 }
diff --git a/src/Components/RedirectException/Subscriber/RedirectExceptionListener.php b/src/Components/RedirectException/Subscriber/RedirectExceptionListener.php
index 0b3cf9bb..08afac28 100644
--- a/src/Components/RedirectException/Subscriber/RedirectExceptionListener.php
+++ b/src/Components/RedirectException/Subscriber/RedirectExceptionListener.php
@@ -14,6 +14,7 @@
 use Ratepay\RpayPayments\Components\RedirectException\Exception\ForwardException;
 use Ratepay\RpayPayments\Components\RedirectException\Exception\RedirectException;
 use Shopware\Core\Checkout\Order\Aggregate\OrderTransaction\OrderTransactionStateHandler;
+use Shopware\Core\Checkout\Payment\Exception\SyncPaymentProcessException;
 use Shopware\Core\Checkout\Payment\PaymentException;
 use Shopware\Core\Framework\Context;
 use Shopware\Core\Framework\Routing\RequestTransformerInterface;
@@ -47,7 +48,10 @@ public function onKernelException(ExceptionEvent $event): void
         $throwable = $event->getThrowable();
 
         $prevThrowable = $throwable->getPrevious();
-        if ($prevThrowable instanceof PaymentException) {
+        if ((class_exists(PaymentException::class) && $prevThrowable instanceof PaymentException)
+            || (class_exists(SyncPaymentProcessException::class) && $prevThrowable instanceof SyncPaymentProcessException)
+        ) {
+            // SyncPaymentProcessException and class_exists checks are required for sw <= 6.5.3 compatibility
             // set the transaction to failed. Without this, the customer will not be able to try a repayment.
             $this->orderTransactionStateHandler->fail($prevThrowable->getOrderTransactionId(), Context::createDefaultContext());
         }