500 Internal Server Error for ONE user after 3.5 upgrade

Does anyone know what might cause a 500 Internal Server Error for ONE user account and no others?

After upgrading to 3.5, one user is unable to access any of the sites after he logs in. He gets a 500 Internal Server Error any place he goes on the front end and in the admin screens.

I have no problem accessing the sites, and neither do several other users. I was able to duplicate the issue myself by logging in as the user who is affected. As soon as I logged in as him, I received the same 500 Internal Server Error.

Clearing cookies and restarting my browser cleared the session for me, which allowed me to access the front end and the login screen again, but logging back in as the user brings back the 500 Internal Server Error.

The PHP log has warnings about wpdb->prepare (which I’m ignoring for now as they’re warnings rather than errors) and some WordPress Database Errors related to one of the plugins. I tried deactivating that plugin, but there was no change in the 500 Internal Server Error for this one user.

I know that I’m supposed to deactivate all plugins and use the default theme, but the sites are live, so I can’t do anything like that until the weekend. I’d be fantastic if I could resolve the issue before then.

The .htaccess file looks normal, but of course, why would a problem with the .htacess file affect just one user? I’ve looked at all the user postmeta in the database. Nothing looks unusual, but then again, I’m not sure what I’d be looking for.