WP Ver: 2.8.4

When attempting up update some users via the admin panel the site is throwing a 500 error. This only occurs with user id's below 138, users with higher id's appear to be unaffected.

Things I've tried so far:

• Compared all files in /wp-admin/ and /wp-includes/ to a fresh copy of wp. They're identical.
• Disabled all plugins. Error still occurs.
• Reverted to default theme (with plugins disabled). Error still occurs.
• Looked through wp_usermeta and wp_users in the db to see if there's something in common among the id's having a problem. Nothing jumped out.
• Also enabled debugging to see if I could get a more detailed error report. Still throws a 500 with no additional info.

I've run out of ideas on how to troubleshoot the issue and haven't been able to find anything through searches. Any suggestions would be helpful.