Hi,
In z-ocsinventory-server.conf file set OCS_OPT_DBI_PRINT_ERROR to 1 and restart apache. With this option database messages will be logged in apache error log file.
Another information : the URL you set in each agent has to be http://@ip_or_fqdn:port_if_different_of_80/ocsinventory and not http://@ip_or_fqdn:port_if_different_of_80/ocsreports
Regards
Frank