I've been working on it and updating it. But have not seen this message. It's not an uncommon error but to track it down, I'd have to know which file it's coming from. Could you describe when it occurs? It sounds as though it occurs on start up. Does it occur if you disable the news plugin? Try downloading the current version.
I've installed this plugin again and I have no error - good. Plugin works, but not in 100% :). First method works. second with:
Doesn't work. It generates blank, empty RSS: http://www.riversedge.pl/newsfeed.php
Maybe because I use other plugin for displaying text - Styler? I removed <style justify></style> tags from text, generated newsfeed again and it's empty.
I've tested it with headers level 1 and 4...