thanks for your patience had until now.
There were two serious errors in the configuration file:
A) the variable logfile was not correct;
b) the server variable must have an address https and not http.
now I have no more errors the agent execution and the log file is written.
Here is the result of the log file:
[Thu Sep 18 15:37:33 2014][debug] Ocsinventory unified agent for UNIX, Linux and MacOSX 2.1.1
[Thu Sep 18 15:37:33 2014][debug] Log system initialised (File)
[Thu Sep 18 15:37:33 2014][debug] --scan-homedirs missing. Don't scan user directories
[Thu Sep 18 15:37:33 2014][debug] Accountinfo file: /var/lib/ocsinventory-agent/https:__localhost_ocsinventory/ocsinv.adm
[Thu Sep 18 15:37:33 2014][debug] OCS Agent initialised
[Thu Sep 18 15:37:33 2014][debug] Turns hooks on for /etc/ocsinventory-agent/modules.conf
[Thu Sep 18 15:37:33 2014][debug] Ocsinventory unified agent for UNIX, Linux and MacOSX 2.1.1
[Thu Sep 18 15:37:33 2014][debug] Log system initialised (File)
[Thu Sep 18 15:37:33 2014][debug] Ocsinventory unified agent for UNIX, Linux and MacOSX 2.1.1
[Thu Sep 18 15:37:33 2014][debug] Log system initialised (File)
[Thu Sep 18 15:37:33 2014][debug] Calling handlers : `start_handler'
[Thu Sep 18 15:37:33 2014][debug] [snmp] Calling snmp_start_handler
[Thu Sep 18 15:37:33 2014][debug] - Net::SNMP loaded
[Thu Sep 18 15:37:33 2014][debug] [download] Calling download_start_handler
[Thu Sep 18 15:37:33 2014][debug] - LWP loaded
[Thu Sep 18 15:37:33 2014][debug] - LWP::Protocol::https loaded
[Thu Sep 18 15:37:33 2014][debug] Compress::Zlib is available.
[Thu Sep 18 15:37:33 2014][debug] Calling handlers : `prolog_writer'
[Thu Sep 18 15:37:33 2014][debug] sending XML
[Thu Sep 18 15:37:33 2014][debug] sending: <?xml version="1.0" encoding="UTF-8"?>
<REQUEST>
<DEVICEID>ubuntu-2014-09-18-15-05-39</DEVICEID>
<QUERY>PROLOG</QUERY>
</REQUEST>
[Thu Sep 18 15:37:33 2014][error] Cannot establish communication : 500 SSL_ca_file /etc/ocsinventory-agent does not exist