> Usually after I mirror a site (and everything is working perfectly), after a
few days, when I try to access the copied site, a 404 error appears. When I
try to find the copied files in the website's folder, they have disappeared.
How can I fix this problem?

You HTTrack a site and it's all OK, and then later some files disappear?  Well
HTTrack can't magically delete files unless you run the program again, so
there must be some other process on your machine doing something...