To make the new simpler config for Apache >= 2.4.47 work, I had to remove the trailing slash in the URL, like this:
ProxyPass /whiteboard http://localhost:3002 upgrade=websocket
Btw, the older configuration for Apache < 2.4.47 still works with newer Apache versions, so maybe cou could also try that, in case the above isn’t working: https://github.com/nextcloud/whiteboard?tab=readme-ov-file#apache–2447