I updated kernel and firmware on two of the raspberry pis in my homelab (total of 6), and now those two nodes have trouble creating sandbox for pods. The errors are complaining that

loadFlannelSubnetEnv failed: open /run/flannel/subnet.env: no such file or directory

, which is correct, there is no such file or directory. I see that it exists on the other nodes in the cluster, so presumably something broke after the kernel/firmware upgrade. What is responsible for creating that file, and where should I start looking for causes? Any suggestions for fixes are welcome.

You're probably missing the vxlan kernel module on the new kernel. It's not included in the default kernel package for Pi. This is covered in the k3s docs.

That sounds plausible. I saw it in the docs, but didn't consider it because I'm not using ubuntu, I'm using DietPi. Of course, I should have realised it could be relevant, because dietpi is based on debian, and it wouldn't surprise me if the vxlan in separate module comes from debian originally. I'll look into it, thanks!