since you cannot see what the error code is then it would be a long shot guess as to what is causing this fault.

i did check in hypass for this truck by the serial number you gave and it did reveal a service gram about a switch issue
(gram number CI00033, SUBJECT: TE-10, TE-27 & TE-32 Transmission Pressure Feedback Switch and Harness)
It says the switch can go bad due to an internal membrane failure causing the symptoms you describe but this does not mean that is what is wrong with it, since we can't see the code we would still be guessing at it.

Your best bet is to call hyster and have them send out a tech to hook up to the machine and see whats going on.