Weird Reflection Rendering to Network

[Robb63]

I sent a turntable animation to our Network Render Farm this weekend, and when I opened up the frames this morning there was a weird reflection in every frame of the animation (see image #1).
It's not in the preview, it's not there when I render locally, and it disappears when I remove the ground plane (and use environment shadows).

I haven't experienced this before with different scenes using the same environment HDRI, and ground plane. Any ideas what might be happening? I've lost about 15 hours of render time with this issue.
Our Network Render manager, and workers are using NR software v 7.1.36