Reinicialize o servidor conforme o comentário de Zacq
Não é a solução que alguém quer ouvir em um servidor Proxmox de produção, mas funciona. Deve haver uma correção de tipo de reinicialização /etc/init.d/[something].
Teve exatamente os mesmos sintomas que o OP.
Além disso, o cliente estava reclamando no dmesg
nfsv3: disagrees about version of symbol nlmclnt_proc
nfsv3: disagrees about version of symbol nfs_alloc_client
nfsv3: disagrees about version of symbol nfs_access_zap_cache
nfsv3: disagrees about version of symbol nfs_submount
nfsv3: disagrees about version of symbol nfs_create_server
nfsv3: disagrees about version of symbol nfs_post_op_update_in$
nfsv3: disagrees about version of symbol nfs_mkdir
nfsv3: disagrees about version of symbol nfs_get_client
nfsv3: disagrees about version of symbol nfs_file_operations
nfsv3: disagrees about version of symbol nfs_lookup
nfsv3: disagrees about version of symbol nfs_getattr
nfsv3: disagrees about version of symbol register_nfs_version
nfsv3: disagrees about version of symbol nfs_setattr
nfsv3: disagrees about version of symbol unregister_nfs_version
nfsv3: disagrees about version of symbol nfs_sops
nfsv3: disagrees about version of symbol nfs_link
nfsv3: disagrees about version of symbol nfs_try_mount
nfsv3: disagrees about version of symbol nfs_close_context
nfsv3: disagrees about version of symbol nfs_symlink
nfsv3: disagrees about version of symbol nfs_clone_server
nfsv3: disagrees about version of symbol nlmclnt_proc
nfsv3: Unknown symbol nfs_writeback_update_inode (err -22)
nfsv3: Unknown symbol nfs_dentry_operations (err -22)