That could be the problem. JuiceboxBuilder-Pro requires AIR v33.1.1.744 (available at the foot of the AIR download page).

Harman removed support for WebKit (the embedded browser within AIR used for JuiceboxBuilder-Pro's live gallery preview on the Customize panel) after v33.1.1.744 so JuiceboxBuilder-Pro requires v33.1.1.744 for the live preview. (Please see this forum post for further information.)

I don't know if this will resolve your problem but it's worth a shot. Try uninstalling AIR v51.1 and install AIR v33.1.1.744 instead. If you don't find an uninstaller in the '/Applications/Utilities' folder, just launch the '/Applications/Adobe AIR Installer' application. Launching this without arguments will put it into 'uninstaller' mode.

If this does not help, then here are a few things that you can do to try to figure out the root of the problem.

(1) Try opening the gallery's 'config.xml' file directly in a browser. If there is a problem with the XML syntax, the browser should let you know the problem and the line at which the problem occurs.

(2) Check to see if any of your image filenames contain special or accented characters. This may prevent them from being opened by JuiecboxBuilder-Pro. If you find any, it would be safer to stick to alpha-numeric characters for filenames.

(3) Try reducing your gallery into chunks of smaller images (by removing image entries from the 'config.xml' file) to see if there is a corrupt image which is preventing the gallery from being opened.

If the problem persists, then maybe you could zip your gallery and send me a link (steven@juicebox.net) so that I can try to open the gallery in JuiceboxBuilder-Pro on my own computer (Windows 11 PC).
Unfortunately, I do not have access to a Mac running Sequoia so the platforms are not the same but maybe I can find a problem with it that is preventing the gallery from being opened.
Thank you.