Thanks for the reply.
Maybe… I’m not super experienced with zfs but I’m not sure the linked issue relates to me. I’m using the same encryption key on both servers and both keys are already loaded.
What is really strange is it does work for some encrypted containers. But I can’t successfully copy any new containers to another server’s encrypted pool using --refresh.
Anyway I give up. I’ll set up LUKS encryption on the backup server and won’t use zfs native encryption. At least it seems to work that way. I don’t need any special features on a backup server but it does need to be reliable, and encrypted at rest, so the above should suffice for now.