[chrishajer]

Did you try validating the feed

http://feedvalidator.org/check.cgi?url=http%3A%2F%2Fwww.criticalmass.at%2Fforum%2Frss%2F

The feed URL returns a 404 Not found Error if you check the headers:

http://www.criticalmass.at/forum/rss/

This sounds like WordPress trying to handle the /forum/ URL and sending a 404 error when the page actually exists. There was a plugin to take care of this sort of thing, I think…

https://bbpress.org/forums/topic/bbpress-wordpress-mu-or-not-leads-to-404-errors-but-pages-still-load#post-13156

I think that plugin will resolve the fact that the feed actually returns a 404 error, then I bet the widget will pull the feed just fine. Report back please…