I've just made tests and it's definitivelly not any hook.
I set RSS cache to 1 minute. Disabled all hooks by define('DISABLE_HOOKS', true); - tested does hooks are dissabled (forum looks

). Checked content of RSS. Wrote new post. Wait a minute - check again does new post is visible in RSS (so cache is refreshed) and then check with option forumids and type=rss2. And in both cases - fail.
As I wrote before for some time it was working again and now again is not working. I'm wondering does the content of posts or some usernames can matter here.
Somebody can help me?...