Continuing the discussion from Private Message Can't Be Reached with private messaging disabled:
Actually (now), on community.metabrainz.org, where Discourse private messages (PM) are disabled because we already have a PM system, when I go to top secret 
my messages hidden link, I can see some interesting content (flagged posts, my comments to admins and their answers):
The fix in the linked topic was to let envelope link to messages active for admins.
But in fact, it seems that it should be active for everyone who has received messages.
The Messages entry should also appear in the Preferences drop down select.