With the help of Bill over at the LivingDot helpdesk, I have managed to fix the problem with the feed from ahistoryofnewyork.com.
Bill reported that when he tried to access the feed, he received an error message: “An invalid character was found in text content.”
At first, I didn’t know how to reproduce the error, so I scanned the entries since August 8 for anything that might look like an invalid character. I made a few changes, but nothing helped. Then I tried to recreate the feed widget with Feeds.App Lite, and the process failed. I was sent over to feedvalidator.org, which identified the word in the entry “FringeNYC” that was causing the problem. It turned out to be the word “Tuesday,” which must have contained some kind of hidden character that wasn’t showing up on my screen. So I deleted “Tuesday,” rewrote “Tuesday,” saved the post and — voila! — everything is working again.
So if you’re having trouble with illegal content in your feeds, use feedvalidator.org to isolate the problem. Thanks again, Bill!