1) open http://localhost/ (DNS entry for the zabbix apache install)
2) enter valid domain credentials
3) now an incompletely-composed main page is sent, detailed far above. This is the version (which only appears when using HTTP auth) that shows 'ERROR: No permissions !', and the footer line shows 'Connected as 'guest'', even after refreshing the page.
4) ONLY after clicking 'Login' from the header (which hits http://localhost/index.php?reconnect=1) does the session fully complete, and result in the user's sent HTTP credentials.