It's a data point though, our NAS environments are different (although both Linux), and we both experience the kerberos issue only with Proxmox, correct? So something with Proxmox' implementation of CIFS, that was not in v6, but started in v7...
Worked around the issue, as I was unable to resolve. Shares used by my Proxmox instance are now all NFS. Are you using unRAID, or do you use an other NAS product?