It sounds like both problems might be connected to each other (or, indeed, be caused by the same issue).
Hopefully my notes below will help.

There are only a couple of things which have been known to prevent an existing gallery from being opened in JuiceboxBuilder-Pro.
(1) If the gallery folder is located within a Google Drive folder.
(2) If you have Avast installed and Avast's Ransomware Shield is active.

#1
Make sure that your gallery folder is not located within a folder which is synced with an online file sharing service (such as Google Drive). If it is, then try copying your gallery folder to a location on your hard drive that is not being synced with an online file sharing service (such as your desktop) to see if the gallery will open from there.

However, before you try this, you may need to do the following to clear the problem:

(1) Close JuiceboxBuilder-Pro.
(2) Navigate to the following location in Windows File Explorer: C:\Users\Username\Documents\JuiceboxBuilder-Pro\
(3) Delete the .tempGallery folder from this location.

I'm not sure what the equivalent location would be for a Mac but if you search for ".tempGallery" you should hopefully find it.

#2
There was a brief problem a few weeks ago whereby Avast's Ransomware Shield prevented an existing gallery from being opened in JuiceboxBuilder-Pro.
This was a false positive result and I contacted Avast who was happy to clear JuiceboxBuilder-Pro's reputation in their database (so there should be no longer be a problem for Avast users as long as their databases are up to date).
If you are not an Avast user, then just in case there is a similar issue with any other security software, if you have any security software installed, try making an exception for the "JuiceboxBuilder-Pro.exe" file to see if this helps.

If #1 and #2 are not relevant or do not help, then I would still try deleting the ".tempGallery" folder from #1 (if it exists) to see if it helps. Also, please try manually deleting the following folder which has also been known to resolve a few odd problems:
Mac: /Users/Username/Library/Application Support/Adobe/AIR/ELS/JuiceboxBuilder-Pro
Windows: C:\Users\Username\AppData\Roaming\Adobe\AIR\ELS\JuiceboxBuilder-Pro

Failing all else, I would try uninstalling and then reinstalling both Adobe AIR and JuiceboxBuilder-Pro.
You can download the latest version of Adobe AIR (v27.0) from here.

Here's the procedure for a clean reinstall of JuiceboxBuilder-Pro:

(1) Uninstall JuiceboxBuilder-Pro.
Mac: Delete the 'JuiceboxBuilder-Pro' file from the Applications folder and empty your Trash.
Windows: Use the uninstaller in the 'Control Panel -> Programs -> Uninstall a program...' list.

(2) After uninstalling JuiceboxBuilder-Pro, please check that there are no files are left behind by manually deleting the following folders (if they exist) from your hard drive:

Mac:
/Users/Username/Library/Application Support/Adobe/AIR/ELS/JuiceboxBuilder-Pro
/Users/Username/Library/Preferences/JuiceboxBuilder-Pro

Windows:
C:\Users\Username\AppData\Roaming\Adobe\AIR\ELS\JuiceboxBuilder-Pro
C:\Users\Username\AppData\Roaming\JuiceboxBuilder-Pro
C:\Users\Username\Documents\JuiceboxBuilder-Pro
You may need to show hidden files to find some of the above locations in Windows File Explorer.

(3) Also, please follow the procedure above (Steps #1 - #2) for JuiceboxBuilder-Lite, too (if you have it installed).
JuiceboxBuilder-Lite and JuiceboxBuilder-Pro can both be installed and run side by side on the same computer but, being that you are experiencing problems, it might be wise to completely clear your system of all JuiceboxBuilder files before trying to reinstall JuiceboxBuilder-Pro.

(4) Reinstall JuiceboxBuilder-Pro following the instructions here.

I hope these notes help (or at least point you in the right direction).