Show your code (web.xml, jsp, etc.)
I found the reason. login.jsp also refers to the same style.css and logo.jpg. These files are also secured, so login.jsp actually didn't use any style and didn't show any picture, but after a successful login showed the last referred file (picture, in this case).
The solution is to move any files used by login.jsp to unsecure location.
Sorry, quite simple mistake. Thank you for your attention,