I tried some more and still got the "are you sure this is a valid URL?" e.g., https://arxiv.org/abs/gr-qc/0702106

Tried https://arxiv.org/abs/1511.06343 and a couple others and got the "failed to render."

Tried cloning engrafo, then installing docker, then building engrafo, then my disk was filling up and decided I'm done with this for now.