Hi Marcos,
Have you tried to get those users to login from different PCs? Use different browsers? If it only happens for 2 users then I doubt that it is an issue with the Apache configuration. Any setting on Apache would effect all users, not just 2.. .right?
Can you compare the HTTP traffic from a user that works and a user that doesn't? Can you see any differences in that traffic?
Sorry I can't help more
Simon