iPod Classic no longer syncing

I have an iMac running Mac OS 12.6.7 (Monterey; it's not eligible for Ventura). I also have an iPod Classic 80GB. Up to about 2 weeks ago I was able to sync Music on the Mac with the iPod. (Ed. to add: Message footer shows the Mac has 10.11, which is not correct).

Yes, I know this is done in Finder now (although the iPod does show up in Music as well, and if I click Sync Settings there, it takes me to the iPod in Finder). Yes, I've set up sync preferences/settings (it's syncing music only).

Again, this used to work flawlessly. It has worked flawlessly, all along, even after iTunes was obsoleted and partitioned into Music, TV, Podcasts, etc. There's also nothing wrong with the iPod; it works just fine to play music, via headphones or connected to my car. It's also worked flawlessly, all along.

Again, whatever changed, happened sometime in the past 2 weeks. Again, this entire scenario worked perfectly ... for years. Yes, that's "years." Years and years and years.

Also, I've made no changes to the Mac or the iPod.

At any rate, I altered some playlists in Music on the Mac, which haven't changed on the iPod, and no amount of connecting it up and clicking Sync in Finder will get it to do so. When I do that, the iPod display will briefly show "Syncing" and then revert to "Connected" (which is its normal behavior). But, nothing changes on it.

Not sure what happened, so wondering if anyone has any ideas. Thanks in advance.