Some more details...

Yes, I used my login data (information deleted from the log file for
understandable reasons...).
I also tried to capture the login URL as described in the help file
<http://httrack.kauler.com/help/CatchURL_tutorial>. But I do not get the
expected return page saying that httrack has captured the data (yes, I changed
the proxy settings...).

Thanks, Axel