Quick Answer: This may be related to you having made a new installation of your operating system without having deactivated the license key for the old installation beforehand.
[Update, 13 Oct 2017: A new license management system is used as per today, which renders the Nalpeiron license manager as obsolete. Fill in your customer data in this form for us to provide you with the latest software build - http://re-compose.com/activate-your-license-for-liquid-notes.html ]
Licensed on a different PCLiquid Notes comes with a licensing manager that handles trial period and activation. Any activation of the license code you purchased is saved in a database in the cloud, and includes a unique set of identifiers for your computer (Installation ID, computer number).
It may happen that, when people upgrade to a new operating system or decide to move their operating system to another hard drive, an error message "Licensing error! This product was licensed on a different PC" is shown. It usually happens if you haven't deactivated your license before proceeding with the reinstallation or move of your operating system, as the system believes the old activation for this computer is still active.
In this case we have to reset the activation on the old system manually for you before you are able to activate a new installation again!. Reach out to our customer support at firstname.lastname@example.org to fix this problem.