The source was not found but some or all event logs could not be searched. Inaccessible logs:Security

To give Network Service read permission on the EventLog/Security key (as suggested by Firenzi and royrules22) follow instructions from http://geekswithblogs.net/timh/archive/2005/10/05/56029.aspx

1. Open the Registry Editor:
   1. Select Start then Run
   2. Enter regedt32 or regedit

2. Navigate/expand to the following key:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Eventlog\Security

3. Right click on this entry and select Permissions

4. Add the Network Service user

5. Give it Read permission

UPDATE: The steps above are ok on developer machines, where you do not use deployment process to install application.
However if you deploy your application to other machine(s), consider to register event log sources during installation as suggested in SailAvid's and Nicole Calinoiu's answers.

I am using PowerShell function (calling in Octopus Deploy.ps1)

function Create-EventSources() {
    $eventSources = @("MySource1","MySource2" )
    foreach ($source in $eventSources) {
            if ([System.Diagnostics.EventLog]::SourceExists($source) -eq $false) {
                [System.Diagnostics.EventLog]::CreateEventSource($source, "Application")
            }
    }
}

I finally found the answer (I haven't noted my source but it was from a search);

While the code works in Windows XP, in Windows 7, you must add this at the beginning:

// using System.Net;
ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
// Use SecurityProtocolType.Ssl3 if needed for compatibility reasons

And now, it works perfectly.

ADDENDUM

As mentioned by Robin French; if you are getting this problem while configuring PayPal, please note that they won't support SSL3 starting by December, 3rd 2018. You'll need to use TLS. Here's Paypal page about it.