It is due to a CORS issue:
Access to XMLHttpRequest at ‘https://open-vsx.org/vscode/gallery/extensionquery’ from origin ‘vscode-file://vscode-app’ has been blocked by CORS policy: Request header field x-market-user-id is not allowed by Access-Control-Allow-Headers in preflight response.
You can see this if you open the developer tools (Help > Toggle Developer Tools) and navigate to the Console/Network tab.