The most likely cause of unresponsive "New Gallery..." and "Open Gallery..." buttons is a persistent '.tempGallery' folder.
JuiceboxBuilder-Pro creates a temporary folder (named '.tempGallery') whilst a gallery is being created or edited. This folder is automatically deleted on closing JuiceboxBuilder-Pro.
However, if JuiceboxBuilder-Pro is unable to delete the '.tempGallery' folder on closing the application, then the "New Gallery..." and "Open Gallery..." buttons may become unresponsive the next time the program is used.
The most likely cause of a persistent '.tempGallery' folder is the use of an online synced file-sharing folder so we do not recommend saving a gallery to (or opening a gallery from) a folder being synced to an online file-sharing service such as Dropbox or Google Drive.
On a Windows system, the '.tempGallery' folder is located here:
With JuiceboxBuilder-Pro closed, please check to see if your own system has a '.tempGallery' folder in this location and, if so, manually delete it before re-opening JuiceboxBuidler-Pro.
Also, try completely disabling any third-party security software that you might have installed (e.g. Avast or AVG), including real-time protection and ransomware detection in case this may somehow be interfering with JuiceboxBuilder-Pro's functionality.
(Please do so at your own risk and be sure to disconnect from the internet first.)
Finally, I know that you say that you have already tried reinstalling JuiceboxBuilder-Pro but it might help to try the complete uninstall/reinstall procedure from this forum thread (Suggestion #12).
I hope this helps.