diff --git a/src/ApiPlatform/ErrorHandler.php b/src/ApiPlatform/ErrorHandler.php
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--- /dev/null
+++ b/src/ApiPlatform/ErrorHandler.php
@@ -0,0 +1,75 @@
+<?php
+/*
+ * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
+ *
+ *  Copyright (C) 2019 - 2024 Jan Böhmer (https://github.com/jbtronics)
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as published
+ *  by the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Affero General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Affero General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+
+namespace App\ApiPlatform;
+
+use ApiPlatform\Metadata\Operation;
+use ApiPlatform\State\ProviderInterface;
+use Doctrine\ORM\ORMInvalidArgumentException;
+use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
+use ApiPlatform\State\ApiResource\Error;
+use Symfony\Component\DependencyInjection\Attribute\Autowire;
+
+/**
+ * This class adds a custom error if the user tries to create a new entity through a relation, and suggests to do reference it through an IRI instead.
+ * This class decorates the default error handler of API Platform.
+ */
+#[AsDecorator('api_platform.state.error_provider')]
+final class ErrorHandler implements ProviderInterface
+{
+    public function __construct(private readonly ProviderInterface $decorated, #[Autowire('%kernel.debug%')] private readonly bool $debug)
+    {
+
+    }
+
+    public function provide(Operation $operation, array $uriVariables = [], array $context = []): object|array|null
+    {
+        $request = $context['request'];
+        $format = $request->getRequestFormat();
+        $exception = $request->attributes->get('exception');
+
+        //Check if the exception is a ORM InvalidArgument exception and complains about a not-persisted entity through relation
+        if ($exception instanceof ORMInvalidArgumentException && str_contains($exception->getMessage(), 'A new entity was found through the relationship')) {
+            //Extract the entity class and property name from the exception message
+            $matches = [];
+            preg_match('/A new entity was found through the relationship \'(?<property>.*)\'/i', $exception->getMessage(), $matches);
+
+            $property = $matches['property'] ?? "unknown";
+
+            //Create a new error response
+            $error = Error::createFromException($exception, 400);
+
+            //Return the error response
+            $detail = "You tried to create a new entity through the relation '$property', but this is not allowed. Please create the entity first and then reference it through an IRI!";
+            //If we are in debug mode, add the exception message to the error response
+            if ($this->debug) {
+                $detail .= " Original exception message: " . $exception->getMessage();
+            }
+            $error->setDetail($detail);
+            return $error;
+        }
+
+
+        return $this->decorated->provide($operation, $uriVariables, $context);
+    }
+}
\ No newline at end of file