diff -u b/core/lib/Drupal/Core/Session/SessionManager.php b/core/lib/Drupal/Core/Session/SessionManager.php
--- b/core/lib/Drupal/Core/Session/SessionManager.php
+++ b/core/lib/Drupal/Core/Session/SessionManager.php
@@ -96,7 +96,7 @@
     $this->connection = $connection;
 
     // Save the PHP session handler in case we need to retrieve session data
-    // that PHP stores before Drupal starts the session.
+    // that PHP stores before Drupal starts.
     $this->PHPSessionHandler = session_module_name();
 
     parent::__construct($options, $handler, $metadata_bag);
diff -u b/core/modules/file/file.module b/core/modules/file/file.module
--- b/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -1076,7 +1076,7 @@
 
     // We prefer the PECL extension uploadprogress because it supports multiple
     // simultaneous uploads. APCu only supports one at a time. PHP session
-    // upload progress only works if the other twe extensions are not enabled.
+    // upload progress only works if the other two extensions are not enabled.
     if (extension_loaded('uploadprogress')) {
       $implementation = 'uploadprogress';
     }
diff -u b/core/modules/file/src/Controller/FileWidgetAjaxController.php b/core/modules/file/src/Controller/FileWidgetAjaxController.php
--- b/core/modules/file/src/Controller/FileWidgetAjaxController.php
+++ b/core/modules/file/src/Controller/FileWidgetAjaxController.php
@@ -13,11 +13,11 @@
 class FileWidgetAjaxController implements ContainerInjectionInterface {
 
   /**
-   * The session manager.
+   * The PHP session handler.
    *
-   * @var \Drupal\Core\Session\SessionManagerInterface
+   * @var string
    */
-  protected $sessionManager;
+  protected $PHPSessionHandler;
 
   /**
    * {@inheritdoc}
@@ -33,7 +33,12 @@
    *   The session manager.
    */
   public function __construct(SessionManagerInterface $session_manager) {
-    $this->sessionManager = $session_manager;
+    if (method_exists($session_manager, 'getPHPSessionHandler')) {
+      $this->PHPSessionHandler = $session_manager->getPHPSessionHandler();
+    }
+    else {
+      $this->PHPSessionHandler = 'files';
+    }
   }
 
   /**
@@ -81,13 +86,9 @@
 
       // Get upload status from the built-in PHP session data.
       $status = [];
-      if (method_exists($this->sessionManager, 'getPHPSessionHandler')) {
-        session_module_name($this->sessionManager->getPHPSessionHandler());
-      }
-      else {
-        session_module_name('files');
-      }
-      session_start();
+      session_module_name($this->PHPSessionHandler);
+      // Fail silently if built-in PHP session handler generates an error.
+      @session_start();
       $prefix = ini_get('session.upload_progress.prefix');
       if (isset($_SESSION[$prefix . $key])) {
         $status = $_SESSION[$prefix . $key];