DietPi sends emails: *** SECURITY information for <hostname> ***

Hi,
since I completed the setup of s-nail I receive the following emails on a regular basis:

*** SECURITY information for pc5-raspi ***

pc5-raspi : Feb  1 08:18:53 : www-data : problem with defaults entries ; TTY=unknown ; PWD=/var/www/html/admin ; USER=root ;

What’s is the issue the system is notifying me about?

THX

Do you have Pi-hole installed? Otherwise this directory should not exist on DietPi systems.
Perhaps some bot has you server on the list to check/brute-force often used admin page locations.

Does /var/log/apache2/error.log or journalctl -t apache2 show similar errors?
In case you could install fail2ban to have those blocked, if appear too often. Otherwise simply ignore. If no real brute-force is attempted (many requests each second) and the dir not even exist, they do no harm.