Strange problem with here2 and CAN

I have set up multiple CubeBlacks with here2 using UAVCAN, and had no issues so far, but a new setup left me puzzled. With exactly the same parameters, the here2 (leds, compass, GPS) will only work on one of my setups if I set brd_boot_delay to 2000 or above.
Now here comes the strange part: if I set brd_boot_delay to 2000 on one of my other setups (where this is not necessary), the here2 stops working.

Switching the here2 GPS units makes no difference, so the fault lies with the cubes. The cubes were purchased from the same source at the same time and the serial numbers are next to each other. Using ArduCopter 4.0.3.

This left me puzzled. Any ideas WHY?