I can across and issue on a friends proxmox/ceph server where after deleing the monitor. We where unable to recreate a new monitor using the GUI due to is erroring out with a message about the monitor address already in use error 500.
To correct this, AFTER fully removing the monitor from all files, running services and being listed in the GUI, ceph -s or config files. you'll need to manually create the new monitor or monitors on each server. The "script" below will give you the needed commands.
#!/bin/bash
monitor_name="CHANGE_ME"
monitor_ip="10.1.1.19"
mkdir /var/lib/ceph/mon/ceph-${moitor_name}
mkdir temp
cd temp/
ceph auth get mon. -o keyfile
ceph mon getmap -o mapfile
ceph-mon -i ${moitor_name} --mkfs --monmap mapfile --keyring keyfile
ceph-mon -i ${moitor_name} --public-addr ${monitor_ip}:6789