The user feeds on Area51 are using an incorrect path. You can see on my page:
The paths for the feed links are using "proposal" where they should be using "user".
Current path for the "user feed' link at the bottom of my page:
The correct path:
An additional problem is that the "responses" feed doesn't work.
On nine sites that I've checked, adding "/responses" to the end of the feed path gives you a feed of the items that appear in your responses tab: