Thank you, Steven.
However, I installed JB again and there was no such request for a password for the "PrivateEncryptedDatak" keychain password (and there wasn't one when I installed it the first time either). So this didn't work. I can't seem to avoid the permission issue this way.
Fortunately, I found the files of my old galleries built with JB v1.5.0 including the app itself. These work on my new M1 Mac, so I will keep using the old version for the time being. I wonder if other users have the same issue, and it would be nice to update JB sometime in the future …