If it is a speed sensor error, they *probably* mean the wheel speed sensor. Usually these should pass the magnet about 5-10mm away; if they're too close sometimes they don't trigger. Wires can also break, or plugs can come *partly* undone just barely enough to cause a loss of contact.
If the system has a throttle, there are some systems that have poorly written error-handling trees that report *that* as a "speed sensor" when it's actually a "speed control" because technically it's still a sensor...In that event you could have a broken ground (or other) wire on the throttle causing a voltage outside the bounds of hte error handler.
It could also, again wiht poor error handling tree, be a motor hall sensor that is used to monitor motor speed for internal system management; that can happen because of overheating, water intrusion, vibration, poor construction from factory, etc.
If "on a trial" means "trials type riding" on rough terrain, any of those could be caused by that.