Two things:

1.) See if the login page actually takes you to another 
server that you didn't specify that HTTrack should grab 
from.  If you tell it to grab www.foo.com and the login 
page takes you to www.bar.com, you'll need to include both 
on your HTTrack initial config screen.
2.) Take a look at the source code (View Source) and see 
how complicated the scripting looks.  HTTrack has some 
limitations that it can't handle really complication 
scripting.