After you updated the db for the password column did you clear those records and register new users for them? If you stored the information as only 20 characters it will not pass the comparison on the login. You must ensure that the full hash of the password is stored when the user registers, and that you are comparing the has of the entered password for that user.
If you have any further problems with this post back with an export of your db structure in an sql file and I can test out your pages to see if there is a problem anywhere.