I have also same issue, qcow2 files on NFS share were created under proxmox 5.1, now I would like to migrate them to Proxmox 7.1, I attached the NFS, started the VM's, it works fine, but live disk migration to DRBD volume isn't possible, not error show, but cancelled. New created DRBD volumes...
I am not sure if I hit a bug so I try to check it before submitting the bug.
I created firewall for VM and the outgoing filter in iptables looks like this:
Chain tap101i1-OUT (1 references)
pkts bytes target prot opt in out source destination