Problem using a serverFn in a route loader

When I use a serverFn in an async loader handler, I get this error in the browser:

renderRouterToString.js?v=a5d97e8c:1 Uncaught (in promise) SyntaxError: The requested module '/node_modules/react-dom/server.browser.js?v=a5d97e8c' does not provide an export named 'default' (at renderRouterToString.js?v=a5d97e8c:1:8)

I don't understand the error-message, or how to fix this... please help