Quick Answer: This is related to you attempting to activate with a license key that has already been used, either because you reached the maximum number of activations or you are trying to activate a second time this computer.
[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 ]
Licensing management on your computerLiquid Notes is equipped with a license manager - provided by Nalpeiron, a licensing management solutions provider - which handles everything in regard to licensing: trial period, reactivation of the latter, online and offline activation / deactivation of a license and more.
When installing Liquid Notes, the license manager is tied to your computer's configuration (i.e. hardware, operating system) to manage the above for your installation of Liquid Notes. Any activation attempt from your machine is then logged in the license management database.
The error message "That License Number has already been used" when trying to activate Liquid Notes is related to having hit the maximum number of activations allowed. This may be a result of trying to activate Liquid Notes on too many computers, or having made a configuration change to your system resulting in the same problem.
Maximum number of activations allowedUse of a license key for Liquid Notes on multiple computers is restricted by the maximum number of activations allowed. Read more in this post about the maximum number of activations allowed in Liquid Notes - http://re-compose.desk.com/customer/portal/articles/694467-can-i-install-liquid-notes-on-more-than-one-computer-.
Each activation of Liquid Notes is logged in the license management database. Should you have reached the maximum number of activations allowed for your license key, this will result in the license manager displaying an error message "That License Number has already been used".
In this case you can opt to manually deactivate Liquid Notes on any other machine where you are using it, and transfer the license to the new machine by completing an activation in the license manager thereafter.
Configuration changes made to your computerYou may also run into the limit for the maximum number of activations allowed as a result of having made a change to your computer's configuration: each activation in the license management database entered is tied to your computer's configuration ('Computer ID').
In case of a failure of your hard disk and replacement of it, your computer's configuration will have changed and result in you making an activation attempt for the same license from the same computer, but using a different 'Computer ID'. If there is no more activation for this license number available, the error message "That License Number has already been used." will be shown.
Get in touch with us to manually delete the entry for your old 'Computer ID' in the database and allow you to activate Liquid Notes again.
Maximum number of activations allowed