Content unavailable after updating Synology NAS PlexMediaServer-1.15.6.1079-78232c603-x86_64.spk

ah, this is just me guessing, but I wonder if the default photo shared folder is another “special” folder like the default video folder.

You could try creating a new folder and putting some of your photos in there as a test, and see if Plex is able to see those.