For security reasons, FollowSymLinks is disabled by default.
To prevent this, either:
1) Remove FollowSymLinks from your .htaccess file. It can be replaced withSymLinksIfOwnerMatch.
2) Or use the patch method instead, which allows FollowSymLinks, but replaces it withSymLinksIfOwnerMatch internally, so clients don't need to adjust anything (we will likely use this as the default in the future).
cd /usr/local/directadmin/custombuild
./build update
./build set harden-symlinks-patch yes
./build set secure_htaccess no
./build apache
./build rewrite_confs