Hello, welcome 🙂
The particular pen lift command that stopped the machine - (C14,END), was that the first pen lift command in the whole queue?
I'm tempted to suggest this might be a power supply issue. The pen lift servo motor runs on the same 5v power supply that the microcontroller (arduino) runs on, so if it draws a lot of current, it can sometimes cause the controller to be starved of power.
The fact that the C14,END command does NOT cause this crash if the servo is not physically attached is interesting, and supports the theory above. But the problem with that theory is that this kind of problem usually triggers a full reset of the board, and I think you'd notice that. If nothing else, it would eventually go READY again.
What do you do when it crashes? Does anything respond? Do you reset it? Do you have to disconnect or reconnect?
How do you have it wired up, and what kind of power supply and motor shield are you using?
When you say "The pen lift and pen drop functions all work fine", do you mean that while the machine is not drawing, you can use the buttons in the controller to send pen lift / drop commands, and they always work?
That's a puzzle!