Hi Roger,
Are you making use of customized self registration portal. In such cases make sure , the session ID of a particular guest login is carried forward to the password change page as well.
For the html changes to any pages (login, aup, self_registration, self_registration_result,
device_registration & change_password) that link back to other pages. The below points A and B should be added as part of customized pages.
A)Reference script (<script src="js/customportals.js"></script>)
B)Add the onsubmit="getDynamicAction(this);" logic for posts
Thanks