diff --git a/src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php b/src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php
index 71d97be8d1..f4711b126d 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php
@@ -179,7 +179,7 @@ EOF
         $preloadFile = $fs->makePathRelative(\dirname($containerFile, 2), $kernelDir);
         $preloadFile .= substr_replace(basename($containerFile), '.preload', -4, 0);
         $preloadFile = var_export('/'.$preloadFile, true);
-        @file_put_contents($kernelDir.'/preload.php', <<<EOPHP
+        @file_put_contents($kernelDir.'/.preload.php', <<<EOPHP
 <?php
 
 if (file_exists(__DIR__.$preloadFile)) {
diff --git a/src/Symfony/Component/Cache/Tests/Adapter/MemcachedAdapterTest.php b/src/Symfony/Component/Cache/Tests/Adapter/MemcachedAdapterTest.php
index 6c9afe04ab..26609ff443 100644
--- a/src/Symfony/Component/Cache/Tests/Adapter/MemcachedAdapterTest.php
+++ b/src/Symfony/Component/Cache/Tests/Adapter/MemcachedAdapterTest.php
@@ -42,11 +42,11 @@ class MemcachedAdapterTest extends AdapterTestCase
         }
     }
 
-    public function createCachePool(int $defaultLifetime = 0): CacheItemPoolInterface
+    public function createCachePool(int $defaultLifetime = 0, string $testMethod = null, string $namespace = null): CacheItemPoolInterface
     {
         $client = $defaultLifetime ? AbstractAdapter::createConnection('memcached://'.getenv('MEMCACHED_HOST')) : self::$client;
 
-        return new MemcachedAdapter($client, str_replace('\\', '.', __CLASS__), $defaultLifetime);
+        return new MemcachedAdapter($client, $namespace ?? str_replace('\\', '.', __CLASS__), $defaultLifetime);
     }
 
     public function testOptions()
@@ -248,4 +248,36 @@ class MemcachedAdapterTest extends AdapterTestCase
         ];
         $this->assertSame($expected, $client->getServerList());
     }
+
+    public function testKeyEncoding()
+    {
+        $reservedMemcachedCharacters = " \n\r\t\v\f\0";
+
+        $namespace = $reservedMemcachedCharacters.random_int(0, \PHP_INT_MAX);
+        $pool = $this->createCachePool(0, null, $namespace);
+
+        /**
+         * Choose a key that is below {@see \Symfony\Component\Cache\Adapter\MemcachedAdapter::$maxIdLength} so that
+         * {@see \Symfony\Component\Cache\Traits\AbstractTrait::getId()} does not shorten the key but choose special
+         * characters that would be encoded and therefore increase the key length over the Memcached limit.
+         */
+        // 250 is Memcached’s max key length, 7 bytes for prefix seed
+        $key = str_repeat('%', 250 - 7 - \strlen($reservedMemcachedCharacters) - \strlen($namespace)).$reservedMemcachedCharacters;
+
+        self::assertFalse($pool->hasItem($key));
+
+        $item = $pool->getItem($key);
+        self::assertFalse($item->isHit());
+        self::assertSame($key, $item->getKey());
+
+        self::assertTrue($pool->save($item->set('foobar')));
+
+        self::assertTrue($pool->hasItem($key));
+        $item = $pool->getItem($key);
+        self::assertTrue($item->isHit());
+        self::assertSame($key, $item->getKey());
+
+        self::assertTrue($pool->deleteItem($key));
+        self::assertFalse($pool->hasItem($key));
+    }
 }
diff --git a/src/Symfony/Component/HttpClient/Exception/HttpExceptionTrait.php b/src/Symfony/Component/HttpClient/Exception/HttpExceptionTrait.php
index 76e650934b..5b7b448307 100644
--- a/src/Symfony/Component/HttpClient/Exception/HttpExceptionTrait.php
+++ b/src/Symfony/Component/HttpClient/Exception/HttpExceptionTrait.php
@@ -60,7 +60,8 @@ trait HttpExceptionTrait
                 // see http://www.hydra-cg.com/spec/latest/core/#description-of-http-status-codes-and-errors
                 $separator = isset($body['hydra:title'], $body['hydra:description']) ? "\n\n" : '';
                 $message = ($body['hydra:title'] ?? '').$separator.($body['hydra:description'] ?? '');
-            } elseif (isset($body['title']) || isset($body['detail'])) {
+            } elseif ((isset($body['title']) || isset($body['detail']))
+                && (is_scalar($body['title'] ?? '') && is_scalar($body['detail'] ?? ''))) {
                 // see RFC 7807 and https://jsonapi.org/format/#error-objects
                 $separator = isset($body['title'], $body['detail']) ? "\n\n" : '';
                 $message = ($body['title'] ?? '').$separator.($body['detail'] ?? '');
diff --git a/src/Symfony/Component/HttpClient/Tests/Exception/HttpExceptionTraitTest.php b/src/Symfony/Component/HttpClient/Tests/Exception/HttpExceptionTraitTest.php
index ac6c30df1f..c2771cc3ae 100644
--- a/src/Symfony/Component/HttpClient/Tests/Exception/HttpExceptionTraitTest.php
+++ b/src/Symfony/Component/HttpClient/Tests/Exception/HttpExceptionTraitTest.php
@@ -22,15 +22,24 @@ class HttpExceptionTraitTest extends TestCase
 {
     public function provideParseError(): iterable
     {
-        yield ['application/ld+json', '{"hydra:title": "An error occurred", "hydra:description": "Some details"}'];
-        yield ['application/problem+json', '{"title": "An error occurred", "detail": "Some details"}'];
-        yield ['application/vnd.api+json', '{"title": "An error occurred", "detail": "Some details"}'];
+        $errorWithoutMessage = 'HTTP/1.1 400 Bad Request returned for "http://example.com".';
+
+        $errorWithMessage = <<<ERROR
+An error occurred
+
+Some details
+ERROR;
+
+        yield ['application/ld+json', '{"hydra:title": "An error occurred", "hydra:description": "Some details"}', $errorWithMessage];
+        yield ['application/problem+json', '{"title": "An error occurred", "detail": "Some details"}', $errorWithMessage];
+        yield ['application/vnd.api+json', '{"title": "An error occurred", "detail": "Some details"}', $errorWithMessage];
+        yield ['application/json', '{"title": "An error occurred", "detail": {"field_name": ["Some details"]}}', $errorWithoutMessage];
     }
 
     /**
      * @dataProvider provideParseError
      */
-    public function testParseError(string $mimeType, string $json): void
+    public function testParseError(string $mimeType, string $json, string $expectedMessage): void
     {
         $response = $this->createMock(ResponseInterface::class);
         $response
@@ -47,12 +56,7 @@ class HttpExceptionTraitTest extends TestCase
 
         $e = new TestException($response);
         $this->assertSame(400, $e->getCode());
-        $this->assertSame(<<<ERROR
-An error occurred
-
-Some details
-ERROR
-, $e->getMessage());
+        $this->assertSame($expectedMessage, $e->getMessage());
     }
 }