Ubercart

Are they asked to log in a second time if the site jumps to an https based address? If so that's happening to me too.

older versions of IE, others with IE via AOL, and others on Macs using Firefox.

For most of those users, their system was caching an older version of the page that they'd loaded before logging in.

Resetting their cache settings -- or, for those who couldn't be remotely walked through doing that, having them hit "reload" from 1 to 3 times -- took care of their issues.

If that's not what you're experiencing, definitely follow up on the hint that Ryan provided -- I just thought I'd bounce the caching "solution" off ya in the meantime.

Good luck.

Base URL in the Settings file maybe?

/**
* Base URL (optional).
*
* If you are experiencing issues with different site domains,
* uncomment the Base URL statement below (remove the leading hash sign)
* and fill in the URL to your Drupal installation.
*
* You might also want to force users to use a given domain.
* See the .htaccess file for more information.
*
* Examples:
* $base_url = 'http://www.example.com';
* $base_url = 'http://www.example.com:8888';
* $base_url = 'http://www.example.com/drupal';
* $base_url = 'https://www.example.com:8888/drupal';
*
* It is not allowed to have a trailing slash; Drupal will add it
* for you.
*/
# $base_url = 'http://www.example.com'; // NO trailing slash!