During upgrade (for instance from 7.6.xx to 8.1.02) the ITSM installer will reset appadmin account, that means the old password will no longer exist. So, if you have only one administrator user (appadmin) try to create another one before the upgrade.

Additional information: after upgrade appadmin will preserve the roles and permissions but the password will be blank so, if you have Cross-Reference-Blank-Password just try to disbale it, restart the system, login with appadmin and without password, set a new password and, reactivate Cross-Reference-Blank-Password and restart again the system.