Use this link [URL="http://connect.garmin.com/feed/rss/activities?feedname=Garmin_Connect - upd8r&owner=upd8r"]connect.garmin.com/.../activities - upd8r&owner=upd8r[/URL].
I replaced the "%3A " in the address by " - " (each character is important).
For you, this workaround works.
However, for my username (as it contains a "@"), I don't see immediately a solution.