I am reluctant to help as I don't believe in auto-posting to forums, however, IF you have permission to do so by the forum owners:
1) check ALL items that belong in the form, there is a hidden field on that page called referer.
<input type="hidden" name="referer" value="http://www.maluchy.pl/forum/index.php?amp;act=Login&CODE=00" />
Perhaps that is also required to prevent logins?
There are methods programmers put in place to prevent just this sort of thing.
Maybe if you post the referer it may work?