Upgraded from Drupal 6 to 7, now login page won't load.

I am trying to upgrade a Drupal 6 website to Drupal 7. update.php ran after fixing some issues (the database ended up with some records with NULL in it's fields)..

Now that it has run fully, I can't seem to be able to log in. It shows the Create New Account and Login links, but the form never shows up.

Could there still be something corrupted in the database that's causing this..?

Any tricks for getting to the bottom of this quicker other than starting over from scratch in Drupal 7..? I've already tried rebuilding the cache and registry in Drush...

If there are known problems that cause issues with upgrading Drupal, maybe an update checker tool to scan the database for such issues should be made..?

I used Navicat to grab the database, but I've never had issues with it before. This website DID originally come from Drupal 5 years ago...

Thanks in advance for any tips anyone can provide..!