Errors with the Apple Help Indexer
Files to be used have to be valid XHTML files and have to verify against the XHTML DTD

From Apple's Authoring Apple Help Document


  you need to create HTML files for your help pages. To ensure that your help displays properly in Help Viewer, your help files should comply with the HTML 4.01 specification. Your main file—which contains the AppleTitle meta tag—should conform to the XHTML 1.0 specification. 


As the main file is XHTML Apple uses the faster XML parser for it, which Uses Objective C classes beginning with NSXML and errors (e.g. non valid or verified text) will raise NSXMLParserErrorDomain errors.