> Hello, I am trying to "mirror" a site in which the main 
page 
> gives internal server error 500, though part of the page 
is 
> available; Httrack stops on error 500, so, since it's the 
> main page, it is not able to bring anything offline.
> Question: is there any way to make HTTrack ignore error 
500 
> and go on downloading?
No - the error 500 also means that the entry point could 
not be found - httrack has then no ways to "find" other 
pages.

Try to change the user-agent (Set options/Browser id) ; 
sometimes the content is different depending on the user-
agent (Mozilla, IE, or another one), generally due to 
css/dhtml differences, sometimes for explicit offline 
browser filtering.

In any cases, ensure that you are using "gentle" mirroring 
settings (limit the bandwidth and the number of connections)