Plesk auto sign out issue with WHMCS + Wordpress

[Xavier12]

Hey guys,

We're having an issue where a Wordpress plugin that integrates WHMCS is not allowing customers to log into Plesk control panel with "Open control panel" without being automatically signed out before even signed in. The page that first appears is https://host.domain.com:8443/index.php then https://host.domain.com:8443/login_...ct_url=https:/%host.domain.com:8443/index.php

Tried the following:
• Changing PHP versions from 5.4 - 5.6
• Changing php session.save location
• New wordpress installation on two different subdomains
• Using nginx only, using apache without nginx
• Restarting plesk services
• Disabling cache such as Memcached and Redis
• Clearing browser history
• Repairing Plesk and reconfiguring nginx and apache on the said sites
• Iframe in wordpress page, WHMCS signs in fine.

"Open control panel" works completely fine when signing in from the root WHMCS subdomain link. So I believe I narrowed it down to it being one or all of the following
• Wordpress
• PHP issue with cookie/session on Plesk

SYSTEM specs
Latest Plesk 12.5.30 Update #29
Cloudlinux 6.7
php 5.5

Please advise, thnx