> After a long time of downloading www.kum.dk, HTTrack 
crashed
> with SIGALRM in the midst of downloading.

?! httrack is not supposed to receive SIGALRM signals ; but 
anyway it should be ignored. (SIGALRM is not trapped in 
httrack)

>  No backtrace
> available in gdb, as the program vanished before gdb could
> grab it.
> Any idea what I should look for?
Absolutely not .. this looks like a stack/heap corruption , 
or a library glitch.
Did you install any upgrade, patch, on the machine ?
Most changes between 3.30 and 3.31-alpha (3.30.9X) concern 
fixes, and I don't really see which one could cause such 
troubles.