As suggested I gradually brought the FOG server and the clients closer together until eventually they were on the same switch/subnet and I’m still encountering the error.
Good work! Although we have not found the root cause of this we did rule out a very important factor.
We’ve also done some more deployments and have found that the issue isn’t as limited to a specific hardware vendor as we initially thought. There have been some recent Dell laptops where we are starting to see this pop up.
So from what we know so far I would guess this is a driver issue in iPXE. Do all the devices showing this issue have the same or a similar network chip (same vendor)?
Try using a different iPXE binary. Which one do you currently use? Are the devices set to UEFI or legacy BIOS mode?