The truck may work fine, then fail intermittently.
Contact a technician to check if your tiller head requires a software/firmware update specifically designed to resolve "implausible signal" errors.
Clear fault & test
: A "Teach-In" (re-calibration) of the potentiometer values is often required to redefine the neutral zone.
: The truck may cut out hydraulics or inhibit travel for safety.
: Ensure the tiller arm is in the correct operating position (not fully upright or fully down) and that no controls (drive/lift) are being touched during startup.