I've got stuck with the same problem; url("aaa") is OK but url(aaa) is not. 
Neither -n or -%P solves it.  

This is definitely an HTTrack bug, since url(aaa) *is* allowed and so many web
sites actually use the format.  

Also, the suggestion ("use the quotes") is simply meaningless if I am not the
site admin.  

I don't think I encountered this when I used HTTrack some years ago.  Maybe a
recent bug?  

CHeeeeeeeeeeers :-)