since yesterday 13:08 european time my apple mail can´t receive the max-forum rss anymore.
error: the rss-feed couldn´t be loaded because the content has an unknown format
(my translation from german).
strangely the jitter rss still works.
I believe the problem is related to individual posts that people make and special characters within those posts. Not every RSS reader is able to parse the special characters, so they complain with a message like the one you’re reporting.