'Request new password' sends a correct email but loses session after login.

I am an experienced Drupal developer, but today I am stumped.
I am testing a community site going live this week. The 'Request new password' link sends an email to the user. The user clicks the email link:
http://my_fake_drupal_site.com/user/reset/3/1252010434/c89730e29595d1152...
and it returns a lovely 'thank you' page that takes you to a Login button. Clicking this Login button gives you the error:
'You have tried to use a one-time login link which has either been used or is no longer valid. Please request a new one using the form below.'

I checked out the recent log entries and I get this series (most recent at the top)

access denied 09/03/2009 - 15:40 user/3/edit Anonymous
user 09/03/2009 - 15:40 Session opened for jjmulletman. jjmulletman
user 09/03/2009 - 15:40 User jjmulletman used one-time login link ... Anonymous
user 09/03/2009 - 15:40 User jjmulletman is on the Reset password ...Anonymous
user 09/03/2009 - 15:40 User jjmulletman is on the Reset password ...Anonymous
user 09/03/2009 - 15:40 Password reset instructions mailed to ... Anonymous

So why does my session open for jjmulletman and then drop session to Anonymous and give me access denied?

Just to get the first few questions out of the way:
*This was built on a test environment Drupal 6.10. Functions correctly.
*Live environment has Drupal 6.13. Had much customization outside of the core. Function is broke.
*I reverted to a canned version of 6.13 plus modules cck, devel, share this. Clean database. All modules are disabled. Function is broke
*Rolled canned version to Drupal 6.10 plus modules cck, devel, share this. Clean database. Function is still broke.

My functioning test cases are on the same dedicated server with the same php/mysql levels.

Am I missing a setting? What would you look at to troubleshoot this?