Thanks for taking the time to reply, esmi, but I'm afraid the answers are:
no
no
no
I've scads of plugins and it would take hours to de- and re-activate all of them just in the hope that it solves the problem. This approach would come into the category of blind hacking. What script outputs the RSS feed, do you know?
I emphasise that this is not a serious problem. RSS feeds are just soooooo noughties, my dear, and I'm not going to spend hours trying to get them working. If I can't identify the problem and fix within an hour or less, then my users will just have to do with our email notifications, Twitter feeds, Facebook posts, listserv posts, and carrier pigeon service. RSS is unimportant, but a nice-to-have.
Fred