To answer the above question I missed - to disable hooks you can edit includes/config.php and put this somewhere in the file (after the <?php line at the top):
Code:
define('DISABLE_HOOKS', true);
But like you said, if you haven't added anything it probably won't be that. You might try disabling zbblock just to make sure, but I don't really see how that could cause this problem.
So that user has a custom avatar? And if you go to the user's profile, can you see it there? I'm kind of out of ideas.