I just found this community while looking for the solution to my problem with HTTRACK, and I've noticed many people before me have had this same problem.
I am trying to download the full contents of a particular site to add to my archives all at once (it would take ages to download all of the individual webpages as PDF files manually) and find it impossible to do it with HTTRACK because it keeps displaying the Mirror Error.
I've tried everything I could already, scoured all the forums and read all of the forum posts on this issue, and nothing that has been suggested has worked so far.