TYPO3 Core

sysext/frontend/Classes/Middleware/StaticRouteResolver.php
return new Response('Invalid route', 404, ['Content-Type' => 'text/plain']);

This triggers a warning in Stream __construct() fopen().

For a basic fix, see #100487, headers might be a topic at other places, too.

There are some further places in core that should be checked. Grep for 'new Response' and check factory usages as well.