The following partial-code reproduces the resulting error for me every time: (Server side code)

[code]

for(var i=0; i<vehicleCivilianCount; i++)
{
    var pedType = PEDTYPE_SPECIAL;
    var model = i == 0 ? driverSkinId : utility.getRandomPedModel();
    var pos2 = new Vec3(pos.x + 1, pos.y + 1, pos.z + 1);

    var civilian = gta.createCivilian(model, pos2, pedType);
    if(!civilian)
    {
        clearCivilians();
        return;
    }

    civilians.push(civilian);
}

var vehicle = gta.createVehicle(vehicleModelId, pos);
if(!vehicle)
{
    clearCivilians();
    return;
}

vehicle.heading = zrotation - utility.radians(90);
vehicle.setData('traffic', 1);

addToWorld(vehicle);

for(var i=0; i<civilians.length; i++)
{
    civilians[i].setData('traffic', 1);
    //civilians[i].setData('vehicle', vehicle); // <--------- This line of code causes the created civilians to not appear. Without this line of code the civilians appear.

    addToWorld(civilians[i]);
}

traffic.elements.set(vehicle, [civilians, vehicle]);

};
[/code]