Apparently, this bug happens when you select "get HTML files first" on some
websites. 

See: <https://forum.httrack.com/readmsg/34270/30846/index.html>

Just un-check that box and try to start new mirror or "update existing mirror"
and see if it fixes it. :)

Hope this helps someone else, since this reply is a bit too late. I was
searching for the same fix and as a very long HTTrack user/lover, I found that
other answer, tried, and it worked. :)

Stay blessed, my friends. :)