Yellow Pencil doesn't load

I love the YellowPencil app, but one site I have doesn’t appear to like it very much and I’m not sure why. If I try to edit the page using the “Edit with YellowPencil” button", it opens a new tab and starts loading, but then stops about 85-95% of the way through, and then a pop-up window appears that says “ says .” and nothing else (see image attached).

I don’t know if it’s a conflict with another plugin or why it’s happening. Is there a place I should start or is this a known issue?