I have the same problem and it seems to me there's a loop in the system. I have proof of being a lifetime customer from 2008 on. And I was always able to install upgrades, up till now. I have a support ticket but there is still no solution. I have just one installation on my pc. And a paid one for my phone, but that's something different. I had to make a new account, because I used to login with my name and a password. Malwarebytes changed that, you now have to fill in your email. When I try to register my license I get the answer my license key exceeded the maximum devices. When I press "deactivate other copies" (I don't have any other, but just in case) the program sends me to my account. There I get the message "You don't have any subscriptions right now..." I press "register a license" and fill in my license key and license ID. Then I get the answer: The product key you are trying to use is already registered... Ugghh?
This is going on for 5 days now. Still no result. Instead of getting an answer today, I got an automated message that I have to follow the steps above. Again.
I'm sure there is a bug in the software. They tiring me out, but I'm not giving up.
Lieke