This sounds like ground you've already covered, but the most common causes of this error message is information that does not jibe at the time of installation with the information given at the time of purchase, or the registration number being entered incorrectly. They must agree.
Suggest copy and paste where the email and registration number are concerned.
The email address entered must match the email address in the download email from MTU. This sounds like "why wouldn't it?", but you'd be surprised at how many times an erronious email was given due to a typo at the time of purchase.
We've even seen cases where a product was ordered from work, and that email address given, then the home one used to try to install...won't work.
Use the email address shown in the download email from MTU, even if MTU made the error. They must agree.
Hope it turns out this simple for you, if not, something's glitched. I'd then try a new download.
If that doesn't do it, then it may take assistance from MTU, but bear in mind they are as human as we, and do take weekends for R&R. Just telling you that to keep the frustration level down in case you have to wait for support
Take care,
George