Windows 2000 active directory, exchange server 2003, outlook 2003, with a mix of XP and Win7 in a mature environment has been working for years without any issue. Out of 100+ users, a random subset of users, some on XP, some on Win7, will get prompted repeatedly for a password to log into Outlook. When the correct password is keyed in, it just prompts again, so they effectively cannot then get in to Outlook at all. Sometimes a close and reopen fixes the problem, sometimes a reboot does, but not consistently, and the problem always reappears eventually, usually within 24 hours. All other users on the same system are fine.
Can't seem to find any commonality among the affected users, except that they all use Outlook. Flat IP network.