Maybe make sure you removed public/all.js (it will regenerate itself) since you did an upgrade..?

Did that, but it didn't seem to make a difference.

Looking at the Firefox console, I see that the webbrowser requests the URL "/so/source/params?spout=spouts_rss_feed" when selecting the RSS Feed source type from the drop down menu, and my web server answers "400 Bad Request" with "no spout type given" in the response body...