Its an issue you having with Joomla. Youre going to have to go to the Joomla forum.
http://forum.joomla.org/
See if using the default template makes any differents.You might also want to try installing a diffrent login module to see if it changes anything. You can download one in the download section of this site.
Heres a post about your problem.
http://forum.joomla.org/viewtopic.php?f=431&t=265750