feed=feed5f6ba6306d185 is invalid XML – Custom Curated RSS Feeds

Hi there, I must be doing something wrong, because the generated feeds all produce the same simple pie error: "feed=feed5f6ba6306d185 is invalid XML, likely due to invalid characters. XML error: Attribute redefined at line 10, column 45" If required, all documented data can be provided. Please advise Thank you and best regards