I just tested this bug on http://demo.moodle.net/
1. log in as admin
2. add calendar block to the home page if not present
3. as you just saw, the pseudo-block "Add a block" works fine
4. hide the calendar block (do not remove it, just close its eye)
5. go back to the pseudo-block "Add a block" and try to add a new block to the page.
The pseudo-block "Add a block" no longer works until you show again the calendar (open its eye).