An UPDATE that may help others
I raised a support form and had a very helpful reply, within an hour, from the Philippines, see the bold text below
Apologies, at this current time the Manage Subscription Portal in CCleaner App and CCleaner webpage is no longer available. That's why even if you put your correct details on it, the page is no longer working and has an error.
In the future, we plan to implement a centralized account that will allow our customers to manage everything applicable to the license, purchase, and account.
They even generated me a link so I can access the "Manage Subscription Portal" of my account.
So the lesson is don't use the email support simply raise a support form, they answer that.
I wonder how many other people have had the same issue as me and been frustrated by not being able to access their Manage My Subscription Portal, surely a message on the log in to the Manage My Subscription Portal stating that it is currently unavailable would save a lot of customer frustration.