I'm building an new DAW and installing all the bits. I saved all my serial numbers and reg codes from previous installations.
I've installed D-Pro 1.5.5, but when I run it, it demands a reg code. 
So I enter the reg code from my text file of codes, and it says "invalid code".
Ok, so I choose "Online activation". The Cakewalk web site does respond, and returns an activation code which is the same as the one I have saved. Still no joy.
 
Then I recalled another post some time back that suggested running "as administrator" could solve this issue.
It did! I have registered D-Pro.

Thanks for reading, and I hope this helps someone else going forward.

That sounds familiar ... been a while though ...

This should be at the top of every Windows troubleshooting list:
 
When in doubt (and are encountering problems), do "Run as Administrator".  Especially true if the installer/application goes back years.