One fix is to modify Argotic.Core/Syndication/Rss/RssEnclosure.cs:
in function
 
public void WriteTo(XmlWriter writer)
 
change
 
writer.WriteAttributeString("url", this.Url != null ? this.Url.ToString() : String.Empty);
 
to
 
writer.WriteAttributeString("url", this.Url != null ? HttpUtility.UrlPathEncode(this.Url.ToString()) : String.Empty);