When I click on the link at the bottom of a Moodle forum post email (just below the text: "To add your reply via the website, click on this link") I get the message: "Sorry, guests are not allowed to post". And I find that I am indeed logged in as a guest.
I don't remember this happening before. Has something changed? Or am I doing something different?
It is now possible to set up a moodle site so that visitors entering courses are logged in as guests automatically. The site from which you received that forum mail must have made use of that new option. Unfortunately the idiot who implemented this new feature forgot to make sure that this would not have side effects like the one you encountered 
. You can guess who that was. He'll now go and fix it.
. You can guess who that was. He'll now go and fix it.