Under certain situations an upgrade could never set a valid OAT cookie. After proceeding to the login page after system check, entering valid details will simply return the user to the login screen with no error message. Entering invalid details will do the same.
This has been verfified on ec2 using a 0.1 config file. A 2.0 config file on the same machine proceeded fine. However at least 1 user has reported this issue with 2.0