I just purchased a license after the 7-day trial. After entering my email and license number into the "Activate TaskPaper" dialog and clicking "Activate License" I get "Sorry, Unspecified error".
The first time this appeared the application had populated the email and license fields automatically.
I uninstalled the trial application along with deleting folders in Application Support, Caches, and the preferences files. I downloaded a new installer from the link that was included with my new license. Same response. "Sorry, Unspecified error".