OS version: ?

I mean the server os, right. But maybe the check is not fine enough for Centos. I will check this, create a issue.
https://github.com/bueltge/Debug-Objects/issues/45

Can you debug for the value $_SERVER[ 'HTTP_USER_AGENT' ].
I check currently this variable and check then '=WIN=', '=NT= and so on.

I just updated the plugin.
$_SERVER[‘HTTP_USER_AGENT’] gives:
Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.99 Safari/537.36

So the problem is this global: it returns wrong info.
I don’t mind btw, because I KNOW what server I’m using ??

But your return values is the point, it is a Windows system –> Windows NT 6.3

Not really…
I’m working on a VPS holding many websites.
I know the difference between WIN and Linux when I’m working with e.g. the terminal.
This is the REAL configuration:

Hostname keesdehaan.info
OS CentOS 6.5 (Final)
Plesk version 12.0.18 Update #32, last updated at Jan 23, 2015 03:31 AM

But how is it possible to get this data, the global $_SERVERhave other data?

I found that the global HTTP_USER_AGENT holds the OS info of the client, not the server.
To get the server OS you can use the global PHP_OS or the function php_uname()
Getting more detailed info on server OS is rather difficult because the version information is vendor specific.
For your purpose I think just the common strings WIN, Linux, BSD, etc given by PHP_OS is good enough.