With "# ceph-mon --monmap /root/monmap --keyring /etc/pve/priv/ceph.mon.keyring --mkfs -i nextclouda -m 10.0.1.1" you created a new MON database (--mkfs) and removed all info from the old one, not only the monmap.
You should have just inserted the new monmap with "ceph-mon -i mon.nextclouda...