I use Dreamweaver and had this problem of not being able to open existing galleries.
The reason was that the  config.xml file was locked, as Dreamweaver makes files write protected if you use the "check in" function in the Dreamweaver FTP panel.

After checking out the a specific config.xml file in Dreamweaver there was no problem opening the gallery in Juiceboxbuilder. : ))

But after failing to open a "checked in" gallery, you might have to restart Juicboxbuilder to be able to open it.

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SUGGESTION TO DEVELOPERS:
There should be a function in Juicboxbuilder, checking if any necessary files are write protected, and if so, Juiceboxbuilder should offer to make these files writable.