There will be quite a few people that have some odd files bookmarked. The default redirectors will cover most of them, but for cases like this they are trying to go to your old login script from 6.5 or prior. What you could do is just replace the contents of that login.php script with a simple redirect to the new login script:

ie:
Code
<?php

header("Location: http://www.domain.com/ubbthreads.php?ubb=login");

?>