That might be a permissions related issue (because of being under the C:\Program Files (x86)\). Please note, that all files bundled with Forest Pack are overwritten during reinstallation / upgrade. For this reason we don't recommend saving any of your custom libraries inside of this library (<forestPackProdir>\lib) and use for this purposes User's Libraries.
In the Library Browser by going to File - Preferences... you can access the Library Browser Options where under Library Paths you can manage folders used for saving your custom libraries.
Hope that helps.