LiveJournal does not support feeds URLs beginning with anything other than "http://". This excludes not only the "feed://" pseudo-scheme but also SSL sites whose URLs start with "https://".
I can imagine that supporting SSL is technically not a problem (I presume that LWP::Simple, or whatever LiveJournal uses to fetch feeds, supports SSL), so this might be either a policy reason (to reduce load?) or an oversight (in which case I presume that suggestions might be the place to turn).
In any event, while the restriction applies, it would be good to document this; the error message, "The URL you have typed is invalid. Please make sure you have typed the URL correctly and try again.", is not terribly informative since https:// URLs look fine to most Internet users.
So I would suggest at least adding this case to FAQ 151 "Why might a syndicated account not update correctly?" (unless you can think of a better place; say, FAQ 137 "How do I watch feeds on my Friends list? What are syndicated accounts?" which explains how to create one?).
Something along these lines, perhaps?
Invalid or unsupport URL scheme: LiveJournal only supports feeds whose URL starts with "http://". If the feed URL you have starts with "feed://", replace it with "http://"; if it starts with "https://" or anything else other than "http://", check to see whether the content is also available from a plain "http://" URL.