Hello Xavier, 
I used httrack for years (many years, sob) on win. Never a slick!
Now I'm definitively on ubuntu, I installed ubuntu several times, to have a
well fitted and stable installation.
In all the previous installations webhttrack worked smoothly! I downloaded
various sites using ubuntu.

Now I get an error instantly, just when I begin the mirroring: and that's
exactly the same error, but a detail: the port is 8081.
It is obviously occupied by some server running for some reason (I didnt
install server deliberately, it has to be some support server for some ubuntu
service).
I'm not able to say to you what is the server, sorry, I havent the ability to
control what synaptic installed as dependent packages, or to scan the ports as
I did in windows (Im a newbie in ubuntu, only a couple of months, with
maaaaaaaany problems :-)

Anyway, please let know why did you choose such important port (8080, tomcat
port) or 8081 (with evidence another important port of some kind I dont
know).

And by side, if you have time, please help me with this problem... :-(
Have to change the 8081 port in some text config file? Where is it?
Thank you very much!!

Looka, from Italy, Turin, (not a fabulous town, actually).