> I guess it's some kind of protection, but if its not that,
> is there anything else I can try?
Not necessaruly - it might be a bogus parsing by httrack in 
case of javascripting. You can try to change the user-agent 
(in set options / browser id) and see if you still have the 
404 errors. But ensure that you are using *reasonnable* 
download settings - and limit the bandwidth, especially 
(set options / limits / max transfer rate)