I noticed this same propensity for cryptic error messages, but same fix works:

When lauching Fetch Art, don't have your entire iTunes "Library" selected (though Fetch Art seems to choose that by default on first launch.)

Instead, choose any Playlist instead. Fetch Art will only try to analyze and fetch art for those items...

Unless I chose this way of using it, I think Fetch Art trys to get info on my video's, radio stations, podcasts, whatever, etc, chokes on unexpected info, and then spits out those error messages, as it is probably assumes music files with some sort of ID tags within ...

So now it seems to be working correctly for me, though I am keeping my old 1.2.7 Installer just in case,,,