Why can't I log in?
If you keep getting the log in form, even after you enter your correct nickname and password, then probably you have cookies turned off in your browser.
Open the settings window of your internet browser, and change the security settings to allow session cookies for this site.
Another cause of this problem may be that your browser doesn't check for updates on the web page. If you have been able to log in once, but never again, than this is probably the case.
Open the settings window of your internet browser, and change the browser history settings to actually check for new versions of web pages.