RSS Feed XML Parsing Error

We are heavily reliant on our RSS feed from our news website out to other websites, however we are now getting an error

This feed does not validate.

line 8, column 44: XML parsing error: <unknown>:8:44: not well-formed (invalid token) [help]

We have used W3C Feed validation service. I have no idea how to fix it or remove that line as I assume the feed is generated from Wordpress. xmlns:media="http://search.yahoo.com/mrss/"xmlns:georss="http://www.georss.org/georss" xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#" >

Nobody would have updated any files except for general plugin updates. Has anyone got some insight into the problem?