dazric wrote:

Hardware profile seems like a pretty good description. My experience (as a Windows user) is that upgrading or reinstalling the OS gives the computer a new name, so then, for Pianoteq activation purposes, you effectively have a 'new' computer.

That should hardly be it. I have often installed my computer with a different operating system. Pianotheq always worked without problems

I think PTQ looks at the MAC-address of the main board and uses that as a reference.

Hardware profile seems like a pretty good description. My experience (as a Windows user) is that upgrading or reinstalling the OS gives the computer a new name, so then, for Pianoteq activation purposes, you effectively have a 'new' computer.

That should hardly be it. I have often installed my computer with a different operating system. Pianotheq always worked without problems

My 20 character activation code does not work. What am I doing wrong? Hope you can help. Thanks.

As I understand it (perhaps poorly) the activation codes are generated based on some "profile" of the hardware.
If you change the hardware then Modartt will need to change the activation code.
There may be a problem if you have left pianoteq activated on hardware that you no longer have, or that no longer works.   
The protocol is that you have to deactivate it on the old machine, then request activation on the new one.

Anyway, Support will probably help you untangle it.

My 20 character activation code does not work. What am I doing wrong? Hope you can help. Thanks.

I cannot tell what you are doing wrong. Modartt support should be your first port of call, not the forum.
https://www.pianoteq.com/support

However here is a page that could help you:
https://www.pianoteq.com//activation_help