Have you solved the problem? I have the same problem.
I have updated to version 2.4 and since then I get the error:
Failed to send Prolog <HTTP Status Code #500>
I have checked the file dbconfig.inc.php and it's correct.
I have checked file z-ocsinventory-server.conf and i's correct.
Starting OCS Inventory NG Agent on Friday, January 12, 2018 12:21:20.
AGENT => Running OCS Inventory NG Agent Version 22.214.171.124
AGENT => Using OCS Inventory NG FrameWork Version 126.96.36.199
AGENT => Loading plug-in(s)
DLL PLUGIN => Searching for Plug-in DLL(s) in folder <C:\Program Files (x86)\OCS Inventory Agent\plugins>
DLL PLUGIN => 0 DLL Plug-in(s) successfully loaded on 0 DLL(s) found
AGENT => Using network connection with Communication Server
COM PROVIDER => Loading Communication Provider <C:\Program Files (x86)\OCS Inventory Agent\ComHTTP.dll>
AGENT => Using Communication Provider <OCS Inventory NG cURL Communication Provider> Version <188.8.131.52>
AGENT => Sending Prolog
DID_CHECK => Read DeviceID <PC40A8F04933BA-2018-01-10-14-16-20> and MACs <40:A8:F0:49:33:BA00:00:00:00:00:0000:00:00:00:00:00> in file <ocsinventory.dat>
COM SERVER => Initializing cURL library for sendRequest
COM SERVER => Using cURL with server authentication
COM SERVER => Disabling cURL proxy support
COM SERVER => Disabling cURL SSL server validation support
COM SERVER => Sending HTTP Post request to URL <http://IP_OCSSERVER/ocsinventory>
COM SERVER => HTTP Post response received <HTTP Status Code #500>
COM SERVER => Cleaning cURL library
ERROR *** AGENT => Failed to send Prolog <HTTP Status Code #500>
AGENT => Unloading communication provider
AGENT => Unloading plug-in(s)
AGENT => Execution duration: 00:00:00.