this is just random guess at this point

Author: ViniTou

src/bundle/Controller/InvalidateTokenController.php

@@ -15,7 +15,7 @@
 
 class InvalidateTokenController
 {
-    public const TOKEN_HEADER_NAME = 'X-Invalidate-Token';
+    public const TOKEN_HEADER_NAME = 'x-invalidate-token';
 
     /**
      * @var \Ibexa\Contracts\Core\SiteAccess\ConfigResolverInterface
@@ -71,7 +71,7 @@ public function tokenAction(Request $request)
 
         $headers = $response->headers;
         $headers->set('Content-Type', 'application/vnd.ezplatform.invalidate-token');
-        $headers->set('X-Invalidate-Token', $this->configResolver->getParameter('http_cache.varnish_invalidate_token'));
+        $headers->set(self::TOKEN_HEADER_NAME, $this->configResolver->getParameter('http_cache.varnish_invalidate_token'));
         $response->setSharedMaxAge($this->ttl);
         $response->setVary('Accept', true);
         // header to avoid Symfony SessionListener overwriting the response to private