PHP only generates error messages like this if password.php file is accessed.
We know there is something (a script or external app) requesting a page once a minute, but we don't know which page (password.php is used by every page in Pi-hole web interface).
Please execute this command to find out what is requesting the page (logout from the web interface before executing the command to avoid false positives):
tail /var/log/lighttpd/access-pihole.log