you could try to ask on phpBB community if someone has an idea. Anyway to enable access log on Apache, you would need to activate it first 
Add following line into your configuration and restart Apache.
CustomLog ${APACHE_LOG_DIR}/access.log combined
Files are filling up on my test system
root@DietPiR5S:~# ls -la /var/log/apache2/
total 8
drwxr-x--- 2 root adm 100 Oct 8 11:23 .
drwxr-xr-x 5 root root 220 Oct 8 11:24 ..
-rw-r----- 1 root adm 655 Oct 8 11:30 access.log
-rw-r----- 1 root adm 405 Oct 8 11:23 error.log
-rw-r----- 1 root adm 0 Oct 8 11:23 other_vhosts_access.log
root@DietPiR5S:~#