Given that you're having trouble getting the version number to update initially - If I were a betting man, I'd still say that not all your files were overwritten.
This isn't a bug that I've ever seen (unless all the files aren't uploaded). And I've done several dozen 6.4.2 upgrades over the past few weeks. It's a new security feature but if there are any old files in the mix - you'll get the result you describe.
For giggles -
DELETE these files:
/editbasic.php
/changebasic.php
/templates/default/editbasic.tmpl
And then re-upload them from your fresh 6.4.2 zip.
Then also - in the admin area - under SQL Query - run this query:
SHOW FIELDS FROM w3t_Users
Study that list and make sure the U_ProfileHash field exists (probably near the bottom) and that the type is varchar(32).
If that still doesn't work - you'll probably need to open a ticket to have your install looked at.
