Folks, this has been resolved by phone between Musictraining and MTUSUPPORT.
The problem turned out to be something we have never seen before.
Musictraining installed under one user (Admin probably), and then tried to run under a different user.
The "Serial Number mismatch..." error appears when you do this.
This is the first time we have seen this, but it is not unreasonable for any number of our users to try and/or do this.
We will look and see if there is something we can do so Hoster will run, even if Windows doesn't want to allow it... or at least not say "SN mismatch" which implies the installation was done wrong on MTU's part.
Another BITE from Microsoft.... 
