An offline investigation has revealed the source of this issue.
The pages in this module were being created by copying and pasting parts of HTML pages that were created by another HTML editing tool. These source pages included siginificant client-side scripting and many HTML comments. When parts of these source pages were copied and pasted into the new page, in some cases the pasted HTML included parts of client-side scripts and parts of HTML comments. This caused the HTML in the new page to be invalid HTML syntax.
The fix was to enable the option "Paste mixed content as text" in the Author preferences area. This strips out the client-side scripting and HTML comment syntax and pastes only the text from the selected source.