Hi Hawksnut,
I have just investigated and apparently the server that handles the registrations was having issues for 4 hours, at the time you attempted to register. It is fixed now. Note that you do not have to purchase a second license anyway. As long as it's for your own use, you can use the same license for PC and tablet and also on different browsers on both. We track IPs for every use of the license keys to ensure there is no obvious abuse, but you can certainly use the original.
So firstly, please try your original license key on PC/tablet.
Cheers.