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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).
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Did all of this and still no-go. I'll have to open a support ticket.
Thanks for the effort!