Recovering From an Invalid License File
 
NoteBook saves your license key information in a file in ~/Library/Application Support/NoteBook/Licenses.  If the license key file ever becomes invalid, NoteBook displays an error message.
Click Go to License Manager... to view your Notebook licenses.

Click “Delete” to delete the invalid license file, then “Add” and re-enter your license key.
Be sure to enter your license key and not your license key’s serial number.  NoteBook won’t accept the serial number in lieu of the license key.
 
 
By The Way...

Why might a license file become invalid?  A license file is unique and becomes invalid when copied.  If you ever restore your home folder from a backup, for example, NoteBook may not accept the restored license file.  Re-entering your original license key allows NoteBook to generate a new and valid license file.