Since the GetSID fails in the psTools I figured I'll test my own Name2SID udf (as can be found in the UDF section). And all those tests pass, which means WMI works fine too.
 Code:
? Name2SID(@USERID)
? Name2SID("Domain Admins")

Function Name2SID(Optional $userid,Optional $domain)
  If $userid = "" $userid = @USERID EndIf
  If $domain = "" $domain = @LDOMAIN EndIf
  Dim $objWMIService
  $objWMIService = GetObject("winmgmts:\\.\root\cimv2")
  $Name2SID = $objWMIService.Get("Win32_Account.Name='"+$userid+"',Domain='"+$domain+"'").SID
  If @ERROR EXIT @ERROR EndIf
EndFunction


So I can conclude the following:
When talking to the AD;
LDAP works fine.
WinNT works fine.
WMI works fine.

So I can't seem to figure out how kixtart is talking to the AD.