Codeigniter session recreated on each page – Plesk

I’ve created a site using CodeIgniter and its build in session class. I’ve uploaded it to a Plesk server, which has given me a demo url of: https://1.2.3.4:8443/sitepreview/http/mysite.com (not actual url, different ip and folder).

A new session is being created on each page view – I can tell because session data isn’t kept between pages, and each time a new page is accessed, the session table get a row added to it.

The session is kept correctly locally, where the cookie config is as follows:

$config['cookie_prefix']    = "";
$config['cookie_domain']    = "";
$config['cookie_path']    = "/";

I’m guessing the issue is with the cookie – I’ve used firebug to check, it appears the cookie IS getting sent to the server, but the server is responing with a replacement cookie.

I’ve tried various settings with the live config, but not managed to get anything to work. The variation I think should work correctly is

$config['cookie_prefix']    = "";
$config['cookie_domain']    = "1.2.3.4:8443";
$config['cookie_path']    = "/sitepreview/http/mysite.com";

but it isn’t!

Any ideas?