your url to the about page should be about.html - so without the .html it is giving you an error.
This page seems to say a setting may fix that:
Or you just change your permalink so that they end in a slash which will make the about page be at /about/index.html - but you can drop the index.html part in your urls and just use /about/.
Try the suggestion in the link above to add the cleanurls setting and see if that works.
This was it! Updating all hyperlinks to .html in navigation.html fed into index.html in the build. Now I’ll explore the link you sent and see if I can hide them. Thanks much!