iTunes 12.6.3.6 stopped downloading app updates; it tries and fails without an error
I have iTunes 12.6.3.6 on High Sierra (10.13.6). For roughly a week, iTunes has failed to download iOS app updates, or new purchases from the store. It can see the store, and can see that updates are available (the list keeps growing), but updating fails. iTunes says, "Downloading...", and then quickly stops. There is no error message.
The same thing happens when I try to download a new app from the store w/in iTunes.
I tried updating iTunes to 12.6.5, but installation failed because it's too old, even when I ignore the certificate error.
I've tried a few usual things like restarting and trashing preferences, but nothing terribly drastic yet. I've seen a couple other people with this problem around the internet, but no one's offered a solution.
Anyone else? Anyone have a solution to this?
iMac 27", macOS 10.13
