For quite some time now, we have been getting sporadic reports of mysterious login problems, mostly with Internet Explorer but sometimes with other browsers on other OSes.
There are a other few known problems with proper fixes, and those include:
a) not able to be logged in to multiple sites on the same domain at once. The fix for this is: <code>ini_set('session.name', '.example.com')</code>
b) login works, but you still appear to be logged out. The fix for this is to just press reload.
c) Users login to www.example.com but don't show as logged in on example.com. The fix for this is to put in an .htacces redirect so that www.example.com automatically becomes example.com. See <a href="http://drupal.org/node/77106">Login doesn't work or must be done twice</a> in the Troubleshooting FAQ.
However, if you have tried to log in to just one site, and even a reload fails to show you as logged in, and multiple host names are not the issue, then you are facing this problem. If you are BOTH a) able to reproduce it reliably and b) are comfortable with editing Drupal files, then please comment on this issue and I will try to help you find a solution.
Help by answering the following:
- What _specific_ browser and OS has the problem? Major version / minor version?
- Does another browser on the same OS have the same problem?
- Does the same browser on a different OS have the same problem?
- Does the browser have any toolbars/spyware that could be interfering?
- Does clearing cookies/history/files help?
- Does a browser reinstall help?
- Do any other troubleshooting measures seem to help?