I wonder if hidden posts appear in RSS?
I have an category of posts that are visible only for logged in users, I use no teasers for posts.

Actually, in my trials the hidden (restricted and/or private) posts are filtered into RSS when appropriate for logged users - with or with the teaser enabled. I tested it with 2.7 and 2.6.5

After changing the login you may need to refresh the browser twice. In some cases (conditions not yet determined/reproduced) I've had to clear the browser cache. Let me know if this continues to be a problem for you.