Without seeing the feed when it had the error I can only speculate that the problem was caused by something else.
HTML in the show notes is acceptable, simply removing it should not have caused this issue. HTML markup will get encoded so it does not cause problems as you are reporting.
I caution relying on FeedValidator.org. That service has not updated in years. It will mark feeds as invalid when they technically are valid, they are just not valid in the way FeedValidator.org expects them. For example, if you use any of the new iTunes tags in the RSS like <itunes:order>, FeedValidator.org will fail. Feeds that are hosted via HTTPS will also fail on FeedValidator.org. There is currently no feed validating service that I recommend as none of them have been keeping up to latest standards and trends in recent years. The best one currently is
www.castfeedvalidator.org, but it also does not handle https:// URLs correctly.
The best way to know if a feed is valid and working is to manually subscribe to it in iTunes desktop.