Thank you for the link! I tried copying the site today by using the tutorial. 
However, when I use the proxy settings I get after clicking "capture URL" I
can't open any internet sites anymore, including the login site or logging in
if the site was already open. According to the walk-through I'm supposed to
get the message "Link captured into HTTrackWebsite Copier...". But all I get
is the following error message: "Proxy server refuses connection" (I
translated that into english - I'm not sure what exactly the english error
message says). What am I supposed to do in this case?