you should not need a password to be able to see the error codes, it sounds to me like your trans (either the reverse clutch packs, or the control valve) is bad, but we would want to take pressure readings on the transmissions test ports to be sure.
you will be calling the local Yale Forklift dealer to get them to reset the password or give you a service level password anyway, might as well get them to check the trans.
as far as seeing the codes, watch the dash as soon as the error is happening.