Hi.
On further investigation, it looks like the problem may be due to opening a gallery located in a Google Drive synced folder.
I have been able to replicate very similar symptoms by opening a gallery from Google Drive: an unresponsive 'New Gallery...' button and an 'Open Gallery...' button which opens the 'Select Gallery Folder...' dialog but does not actually open a gallery.
To clear the problem (on a Windows 10 PC), the solution appears to be to close JuiceboxBuilder-Lite and manually delete the following folder:
C:\Users\Username\Documents\JuiceboxBuilder-Lite\.tempGallery
(On a Mac, search for .tempGallery, make sure the search result is inside a parent folder named 'JuiceboxBuilder-Lite' and delete the folder.)
Also, to avoid this problem, I would recommend that you open your galleries from folders that are not synced to online file sharing services (such as Google Drive). If your gallery folders are stored on Google Drive, you could perhaps copy them to your desktop before opening them in JuiceboxBuilder-Lite.
A quick question for you:
Could you please let me know if your problem occurred after trying to open a gallery from Google Drive?
The more information we can gather regarding this issue, the better.
Many thanks!