Some users can't log in

Hi everyone,

I've encountered this really annoying problem I can't seem to get rid of. Some users, after registering (no admin approval is required), can't log in. According to the experience of one of them, the page simply refreshes after clicking the login button.

Now, the weird part is that every time they try to log in, their (default) avatar is displayed in the "Who's online" block. That is, if they try to log in 5 times, there will be 5 avatars of the same user in the WO block. When I check the user list via /admin/user/user, the "Last access" field for the newly created user says "never". It's a new website, and there's 25 members (approximately 7%) in the user list that have never accessed the website. Only two of them have reported the problem, and, according to their experience, both of them have tried logging in via Google Chrome.

Currently, my Drupal installation chooses whether to display the page for anonymous users or the logged in ones via template.php:

function phptemplate_preprocess_page(&$vars) {
  global $user;
  if ($user->uid) {
  	//...skipping unnecessary part

		else{
			$vars['template_file'] = 'page';
                
                //...skipping unnecessary part
		}
	 }
  }
  else {
  	//...skipping unnecessary part
	else{
    	$vars['template_file'] = 'page-anonymous';
	}
  }
}