Check status cloudflared service via Monit. https://www.reddit.com/r/pihole/comments/b94dcr/cloudflare_dns_over_https_doesnt_work_after_a/

clouflared_check.sh

#!/bin/bash
TEMP_STATUS="/tmp/cloudflared.status"
TEMP_ERR="/tmp/cloudflared.err"
TEMP_LOCK="/tmp/checkCloudflared.lock"

lockfile -r 0 $TEMP_LOCK || exit 1

sudo systemctl status cloudflared > $TEMP_STATUS
cat $TEMP_STATUS | grep "failed to connect to an HTTPS backend" > $TEMP_ERR
cat $TEMP_STATUS | grep "failed to perform an HTTPS request" >> $TEMP_ERR

if [ -s $TEMP_ERR ]
then
    rm -f $TEMP_STATUS $TEMP_ERR $TEMP_LOCK
    exit 1
fi

rm -f $TEMP_STATUS $TEMP_ERR $TEMP_LOCK

monit-cloudflared.conf

check program cloudflared with path "/bin/bash -c '/usr/scripts/cloudflared_check.sh'"
    start program = "/bin/systemctl start cloudflared"
    stop  program = "/bin/systemctl stop cloudflared"
    if status = 1 then restart
    if 1 restart within 1 cycle then exec "/bin/bash -c '/usr/local/bin/slack-webhook.rb'"