I think its because your old feed manager was made back in a very old version before we moved the FeedManager to the mojoPorta.Features project.
It can be fixed I believe if you find the feed url for that feed in the Administration > Advanced Tools > Url Manager.
The friendly url for that existing feed probably points to /RSSAggregate.aspx?pageid=x$mid=y
You should change it to point to /FeedManger/FeedAggregate.aspx?pageid=x&mid=y
Hope it helps,