Did you try to change the password and disable expiry date as described in the KB?
The Root PW was not expired.
Did you ever find out that cause for this error? I have encountered the same problem going from 6.0 U3e to 6.5 U1g. These builds were released on the same day, so I wouldn't think it would be a "back in time" conflict.
None of my appliance root passwords are expired or set to expire.
I also tried upgrading to 6.5U2c, and got the same error. Definitely not a "back in time" issue, then.