Tested and reproduced in 6.1, branch, and trunk (revision 116817)
Issue discovered while investigating a different issue.
1. add 2 web contents and link them as related assets
2. add related asset/asset publisher portlets and select one of the assets in the asset publisher portlet
3. select configurations for the Related Asset portlet and enable RSS
4. select an asset in the RSS
Result: "not found" page error