While mechanical binding in the linkage is a possibility i would say it would be unlikely due to the fact that you are loosing other electrically controlled functions like the hydraulics for instance, not to mention the related error codes.

That is why i am asking you to look at the seat switch input circuit and also the park brake circuit. On hysters and Yale lifts alike they both require both of these functions to be in a certain state for the truck to run normally, if they are not the trucks SRO (static return to off) system will time out and shut off the trucks systems thinking the truck is left sitting idle with no operator.
This is a safety feature built in to keep the machine from making any uncommanded movements in lift or drive systems.
This why you are loosing drive, hydraulic and steering ability but the trucks power is not actually turning off. Your display still shows the truck powered on when this happens but all functions are disabled.
At least from your comments thats what appears to be happening.