LESSON:

Create backups.

I save a new version under a new name (dateStamp yymmdd) every day when I start work on a project and sometimes midway through a day if I plan to try something different. Finale files are not that large and so do not use a lot of memory.

Yes! I have Dropbox, which lets you rewind to past versions, and Mac's Time Machine. Sometimes I let Finale save backups automatically every 15 minutes, although in the past this was a source of corruption.

It's not that I can't go back to the last good version. It's that:

1. I am having trouble correcting the file due to error -15

2. Without knowing what caused the problem I fear it will recur

All minus error reports are generated by the MacOS. They are not the actual errors, only reports that something is wrong and many of them are hard to guess. In fact, -15 does not show up on the Carnegie Mellon Mac Error Code list — fun, huh?

https://www.cs.cmu.edu/afs/cs/user/lenzo/html/mac_errors.html 

A developer database I have access to gives us

-15   seOutOfRange            from SetEntry

which is no help at all.

A few things:

Your current work files should be in the Default location in Users/(your name)/Documents/Finale Files. Do not store them on an external drive over USB — that connection sometimes goes to sleep and doesn't wake up properly (although Errors -7 and -43 are more common when that happens). An external over Thunderbolt is rock solid but it's best to point to the folder with a SimLink. So called "Thunderbolt compatible" over USB-C is marketing nonsense — actual TB is licensed and certified by Intel using better components. Archiving old projects on a USB external is fine, of course.

Have you checked your Backup Files? See if you can navigate to them on your System drive: Users/(your name)/Documents/Finale Files/Backups. Finale has had trouble here and  being able to navigate to it can sometimes clear the problem without doing anything else. I had that issue with 26 over Big Sur. If you have multiple versions of Finale installed, it can get confused as to what goes where.

Apple has made changes to Cloud services in Monterey and Ventura. This has a big impact on iCloud and Dropbox—actually an improvement but a lot of users don't like it and very few understand it. If using Dropbox on either OS, I can help you there. That tutorial and discussion deserves its own thread. 

Finale AutoSave has been behaving for a number of years now.

Do you know how to restore individual documents in Time Machine? If not, I'll do a quick tutorial. If within the last 24 hours, fast and easy. Ventura is a little different and not as smooth, however, and Apple needs to fix that.

Without knowing anything more about your setup, my basic recommendation is to completely (in depth) remove all versions of Finale from your Mac. This should not impact your Finale Files folder but I would Archive (zip) the folder anyway and move that to your desktop.

https://makemusic.zendesk.com/hc/en-us/articles/115003352327-Uninstalling-Finale#:~:text=Each%20version%20of%20Finale%20has,a%20good%2C%20basic%20uninstall%20process. 

Download your latest version and do a clean install to your Mac. Make sure that your work files and backups are still in the correct locations.

Lastly— Shut down. Boot into Safe Mode. Log into your Users folder when prompted. When all of your desktop folder icons have rebuilt, go ahead and Reboot. This rebuilds all of your System and Application paths and restores all caches to Default. Intel and Apple Silicon now do this differently:

https://support.apple.com/en-ph/guide/mac-help/mh21245/mac#:~:text=Press%20and%20hold%20the%20power,The%20computer%20restarts%20automatically.