It is possible to enter, e.g. into a forum post, Unicode control characters such as U+0001.
Within XML output, only the control characters 9, 10, and 13 are permitted. Presumably for this reason, the RSS feed output for the forum does not work if somebody enters those characters.
A suitable fix would be to make the Moodle clean_param function capable of stripping out these characters (any control character other than 9, 10, 13).