Extract from output:
javadoc: error - An internal exception has occurred.
(javax.lang.model.element.UnknownElementException: Unknown element: jdk.javadoc.internal.doclets.toolkit.DocFileElement@6b456fb6)
Please file a bug against the javadoc tool via the Java bug reporting page
(http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com)
for duplicates. Include error messages and the following diagnostic in your report. Thank you.
javax.lang.model.element.UnknownElementException: Unknown element: jdk.javadoc.internal.doclets.toolkit.DocFileElement@6b456fb6
at java.compiler.interim/javax.lang.model.util.AbstractElementVisitor6.visitUnknown(AbstractElementVisitor6.java:125)
at jdk.javadoc.interim/jdk.javadoc.internal.doclets.toolkit.DocletElement.accept(DocletElement.java:75)
at java.compiler.interim/javax.lang.model.util.AbstractElementVisitor6.visit(AbstractElementVisitor6.java:106)
at jdk.javadoc.interim/jdk.javadoc.internal.doclets.formats.html.TagletWriterImpl.createAnchorAndSearchIndex(TagletWriterImpl.java:463)
at jdk.javadoc.interim/jdk.javadoc.internal.doclets.formats.html.TagletWriterImpl.indexTagOutput(TagletWriterImpl.java:119)
at jdk.javadoc.interim/jdk.javadoc.internal.doclets.toolkit.taglets.IndexTaglet.getTagletOutput(IndexTaglet.java:52)
at jdk.javadoc.interim/jdk.javadoc.internal.doclets.toolkit.taglets.TagletWriter.getInlineTagOutput(TagletWriter.java:287)
at jdk.javadoc.interim/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter$2.visitIndex(HtmlDocletWriter.java:1480)

The trigger is using {@index} inside an HTML file in a doc-files subdirectory.