I have some news about permissions issues:
I set up a fresh modx installation on another host and followed as usual the basics steps to make member-only pages
http://rtfm.modx.com/display/revolution20/Making+Member-Only+Pages and.. it worked! When trying to access member-pages as anonymous users, it automatically redirects me to the public home page, as expeted, while allowed users can see the page properly.
Back to my website, I re-tried to set permissions and ACLs following this basic configuration and... it doesn't work! It is driving me crazy!!!
I've also noted that when attempting to flush all sessions, an alert window is raised in manager with the message "flush_sessions_err" and nothing else. No messages in Modx Logs. It could be that new permissions aren't flushed properly and not applied at all? What this error is related to?
PS Since FileDownloader R it's not compatible with sqlsrv, I'm using FileLister with some changes I needed and it works like a charm!