Mostly just a guess, but probably your model matching chose a model for one of the nearby aircraft, and that model doesn’t support one of the functions that vPilot calls to modify the appearance of the model, such as lowering/raising the gear or flaps, or one of the lights, etc.
Reducing the number of displayed aircraft probably caused that model to not be shown anymore, thus eliminating the error.
Were you able to just ignore the error, or was it actually causing vPilot to crash?