Just another extract of the error log from Sonarr, if this helps.
Request Failed. POST /api/release: Syntax Error after '<!' characters. http://www.newzbin.com/DTD/nzb/nzb-1.1.dtd Line 2, position 4.
System.Xml.XmlException: Syntax Error after '<!' characters. http://www.newzbin.com/DTD/nzb/nzb-1.1.dtd Line 2, position 4.
at System.Xml.DTDReader.CompileDeclaration () <0x00373>
at System.Xml.DTDReader.ProcessDTDSubset () <0x00127>
at System.Xml.DTDReader.GenerateDTDObjectModel () <0x00267>
at Mono.Xml2.XmlTextReader.GenerateDTDObjectModel (string,string,string,string,int,int) <0x00337>
at Mono.Xml2.XmlTextReader.ReadDoctypeDecl () <0x001b3>
at Mono.Xml2.XmlTextReader.ReadDeclaration () <0x0009b>
at Mono.Xml2.XmlTextReader.ReadContent () <0x0019b>
at Mono.Xml2.XmlTextReader.Read () <0x00237>
at System.Xml.XmlTextReader.Read () <0x000b2>
at Mono.Xml.XmlFilterReader.Read () <0x0001a>
at Mono.Xml.XmlFilterReader.Read () <0x0001a>
at Mono.Xml.XmlFilterReader.Read () <0x00087>
at System.Xml.Linq.XDocument.ReadContent (System.Xml.XmlReader,System.Xml.Linq.LoadOptions) <0x0011e>
at System.Xml.Linq.XDocument.LoadCore (System.Xml.XmlReader,System.Xml.Linq.LoadOptions) <0x00047>
at System.Xml.Linq.XDocument.Load (System.Xml.XmlReader,System.Xml.Linq.LoadOptions) <0x000af>
at System.Xml.Linq.XDocument.Load (System.Xml.XmlReader) <0x0000f>
at NzbDrone.Core.Download.NzbValidationService.Validate (string,byte[]) <0x000af>
at NzbDrone.Core.Download.UsenetClientBase`1<NzbDrone.Core.Download.Clients.Sabnzbd.SabnzbdSettings>.Download (NzbDrone.Core.Parser.Model.RemoteEpisode) <0x00657>
at NzbDrone.Core.Download.DownloadService.DownloadReport (NzbDrone.Core.Parser.Model.RemoteEpisode) <0x004c5>
at NzbDrone.Api.Indexers.ReleaseModule.DownloadRelease (NzbDrone.Api.Indexers.ReleaseResource) <0x000c9>
at NzbDrone.Api.Indexers.ReleaseModule.<.ctor>b__7_0 (object) <0x0002b>
at (wrapper dynamic-method) object.CallSite.Target (System.Runtime.CompilerServices.Closure,System.Runtime.CompilerServices.CallSite,System.Func`2<object, object>,object) <0x000a0>
at Nancy.Routing.Route/<>c__DisplayClass4.<Wrap>b__3 (object,System.Threading.CancellationToken) <0x00137>