The driver version is 440.82 and CUDA version 10.2.
I think the drivers are the problem, since I had the same problem on any machine that uses these drivers (some of them have V100s instead of 1080s).
However, I was able to get multi-GPU training running with the same driver version on another machine when I installed the driver via PPA instead of installing from NVIDIA directly (as it was done on the other machines).
Thanks for your suggestion to look at the drivers.
I’ll report if reinstalling on the other machines doesn’t solve the problem, but for now I consider this problem solved.