This is a handy link and list of all the relevant Directadmin log files and related servers.
http://help.directadmin.com/item.php?id=11
DirectAdmin:
/var/log/directadmin/error.log
/var/log/directadmin/errortaskq.log
/var/log/directadmin/system.log
/var/log/directadmin/security.log
Apache:........