other_vhosts_access.log not working properly
There is an issue with apache log files in ispconfig, where all vhost accesses are logged twice. Once in /var/log/ispconfig/httpd/*/access.log and again in /var/log/apache2/other_vhosts_access.log. I only noticed this recently, when we started having diskspace issues, so I don't know for how long this issue exists.
I believe the problem is caused by the following (default) setting in apache2.conf:
Define an access log for VirtualHosts that don't define their own logfile
CustomLog /var/log/apache2/other_vhosts_access.log vhost_combined
As the comment says, this log should only include vhost accesses that don't define their own logfile, but with ispconfig that seems to be all of them. I suspect, this is due to the fact that ispconfig's CustomLog directive is defined in a separate file (/etc/apache2/sites-available/ispconfig.conf) and not in each vhost file. This issue would probably be resolved if the CustomLog directive is moved from apache_ispconfig.conf.master to vhost.conf.master.