Other Switch to HTTPS - External login/registration issue


I'm trying to switch my website and forum to HTTPS (SSL) usage.
Both separate are working fine, but combined I find some issue's.
Is there someone that can help me on why this is not working properly?

First let me describe what I have now:
A website which is currently working via HTTP.
Users can register and will be registered on the forum automatically.
Also when they login on the website they don't have to login separatly for the forum.

What I want:
Using HTTPS (SSL) for a more secure connection.

Problem I face?
When using HTTPS in the URL a user can register on the website, but isn't automatically registered on the forum.
Also when login on the website the user won't be logged in into the forums.

Is there a way to make this work?
The scripts used can be found in the following topic: https://xenforo.com/community/threads/switch-to-https-external-login-registration-issue.122814/

I'm looking for someone that can help me with this, let me know!