Hi,
just wanted to share my experiences with that problem. Archive logs had permissions for nagios user and group adm. Here it was sufficient to just change the group to www-data. But that was not enough for the logfile /var/log/nagios/nagios.log. It changed nearly instantly back to nagios:nagios with 640 permissions.
I solved it with adding www-data user to group nagios but that seems an odd workaround for me.