Having similar issues. Everything was fine, now as of this Friday nothing will download due to similar error in this post.
“[v2.0.0.5163] System.Xml.XmlException: Syntax Error after ‘<!’ characters. http://www.newzbin.com/DTD/nzb/nzb-1.1.dtd Line 1, position 3.”
I’m running Sonarr inside a docker on top of unraid. Doesn’t appear anything has updated or changed recently, not sure why this is occurring all of a sudden. Will continue to watch here for updates…