Reproduction of issue https://github.com/sveltejs/kit/issues/14684

Steps to reproduce

• git clone [email protected]:jackgdll/sveltekit-promise-streaming-error.git
• cd sveltekit-promise-streaming-error
• pnpm install
• pnpm dev
• Open http://localhost:5173/page/subpage
• The connection to the server is reset instead of showing the error page
• Doing one of the following will restore the expected behaviour:
  • await the promise in ./src/routes/page/+layout.server.js instead of streaming it
  • pnpm i "@sveltejs/kit@<2.39.0" and restart the server
  • move ./src/routes/page/+layout.server.js to ./src/routes/+layout.server.js