Hi There have been various postings on this list concerning "the cannot login" topic. Almost always, there have been cookies involved in the discussion. I just wasted serveral hours with yet another cookie problem and I want to share the solution that worked for me.
On firefox 3, I loaded the cookieculler extension (CC), I nice extension that let you selectively kill or keep cookies. Due to some misconfiguration , the site serving my Intranet drupal (say 192.168.1.2) got kept by cookie culler (there is an option that let CC prevent any site from installing a cookie that has already been deleted).
I noticed that error (was my error, not CCs) that prevented me from login into drupal from that particulaer firefox installation. Other browsers on the same computer and on other computers worked fine. Drupal always returned to login and password field blank with no message (in my opinion, this is really a drupal bug , there must be some message saying that you have to enable cookie to use drupal, as this is the standard on the Internet)
I changed CCs prefs => KO I disabled CC => KO I uninstalled CC => KO I deleted remaining CC files => KO Of course , I always cleared private data =>KO
The solution was:
In the menu Preferences->Privacy->Cookie->Exceptions there is a list of sites that are always blocked from setting cookies, and my drupal site got in there as well.
Unblock my site => OK
Conclusion: If you experience login problems, DO NOT rely on clearing all private data (cache, cookies, etc). Also check the list of sites that are always blocked from setting cookies.
Hope that helps P